Unresolved Form ID errors signify records that try to use non-existing records in the same or master plugins. Certain unresolved Form ID errors prevent successful or safe LOD patch generation and/or can cause problems in the game ranging from something simply not working to CTD.
This can happen due to outdated plugins or if records being used by other records are removed without taking care of such dependencies.
As explained in the Generation Instructions, finalize the load order, install mods, sort and resolve conflicts. Clean and error check the load order with xEdit. Clean every plugin that LOOT suggests to clean - ignore misinformation that cleaning any plugins is not needed or causes problems. While ITMs are irrelevant in this context, see There are no intentional ITMs. The Batch Plugin Cleaner for Mod Organizer or the xEdit Cleaning Extension for Vortex help to automate some of that. Check the load order for errors with xEdit before generating LOD. Fix all errors. See this video for help (ignore the old cleaning instructions as that was already done). Following proper modding practice and error checking with xEdit finds and fixes unresolved Form ID errors long before using tools that require an error free load order. Fixing these errors in the load order means a more stable and better working game and mods.
Start by checking the plugin and record with xEdit mentioned in the Error in ... message.
In case the message mentions that a plugin might be the wrong version, double check it is the correct version for the game version and for any other plugins using it as a master and vice versa. For example, do not use plugins or paid mods from the Creation Club Anniversary Edition with outdated vanilla game plugins from before the Anniversary update (only downgrade the executable of the game, not the plugins). Update the required plugins and use matching versions. Also, after game updates, do not forget to update the cleaned versions of Update.esm, Dawnguard.esm, HearthFires.esm, Dragonborn.esm etc. which might be installed as a mod. For example, patch plugins might need updating in case any of the plugins they are patching changed.
If unresolved form ID errors happen in the latest and all updated vanilla game files (including DLC and CC), consider using the Unofficial Skyrim Legendary Edition Patch, Unofficial Skyrim Special Edition Patch and/or Unofficial Skyrim Creation Club Content Patches.
If unresolved form ID errors happen while modding Skyrim VR - especially when the error mentions Update.esm or DLC might be the wrong version - the error is most likely caused by a plugin that was made for Skyrim or Skyrim Special Edition. Creating a patch with Skyrim VR - USSEP 4.2.2 and SSE 1.5.97 Compatibility Patch to include updates from Skyrim Special Edition and the Unofficial Skyrim Special Edition Patch 4.2.5b should help in those cases.
If unresolved form ID errors happen while modding Enderal or Enderal SE, the error is most likely caused by a plugin that was made for Skyrim or Skyrim Special Edition. Enderal or Enderal SE do not have any data in the DLC plugins and only a rudimentary Update.esm without any of the actual records that exist in the vanilla Update.esm of Skyrim Special Edition. Even the Skyrim.esm from Enderal does not have all the same records as the one from Skyrim. Even worse, a plugin made for Skyrim can overwrite completely different records in Enderal. Plugins depending on records found in vanilla Skyrim plugins need to thoroughly checked, updated and fixed for Enderal in order to work correctly.
In case the unresolved form ID is for a script property and the intended record can not be determined or the property has been removed from the script, remove the property from the properties listing via right click, Remove in xEdit.
In case the unresolved form ID happens in a list of items (for a container like a chest or barrel for example) and the intended record can not be determined, remove the item from the list via right click, Remove in xEdit.
If xEdit shows that a plugin with an resolved form ID overwrites a record in another plugin that has a working form ID, then dragging and dropping the working form ID entry to replace the unresolved form ID can be a solution. Changing the overwrite order of plugins, so that the error free plugin is winning, might work as well. Removing the entire overwriting record with the unresolved form ID from the winning plugin will keep the plugin overwrite order intact. These are just a couple examples how to possibly fix the error when not knowing the initial intention of the mod author(s). Verify that using these solutions does not have undesired effects.
Unresolved form ID errors can cause the game to CTD, prevent mods from working properly and make it impossible to create the LOD patch plugins.
If removing a plugin lets the process run through without issues, it means the plugin has errors that need to be fixed.
Temporarily disabling plugins is a troubleshooting step and not a fix. Avoiding or silencing an error message does not fix the error.
Ignore suggestions to only temporarily disable plugins to complete the process or claims that a plugin with errors is OK to be used.
Fix the error or permanently or remove the problematic plugin. Such plugins and badly made mods should not be used in the game.
Make use of the official DynDOLOD support forum in case help is required to identify or to fix errors.