Errors to copy a record with an Assertion failure might be caused by broken plugins.
Broken means the data in the plugin is corrupt or somehow wrong and can not be processed by xEdit/DynDOLOD.
The error is often caused by a broken bashed patch. Use the latest Wrye Bash version from the #wip-builds channel of the Wrye Bash Discord to create the patch or do not use the CELL patcher.
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).
The cleaning and error checking will address typical errors caused by editing or using the Creation Kit, it does not test for wrong record data caused by bugs in specific Wrye Bash versions.
Look up all the records from the error message by their form ID in xEdit. Typically the error is triggered by the last plugin to overwrite one of the listed records - even if everything appears to look OK on first glance.
If removing a plugin lets the process run through without issues, it means the plugin might be broken.
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 the plugin is fine to use.
Fix the error or permanently remove the broken plugin. Broken plugins should never be used in the game.
It might be possible to fix a broken plugin by loading and saving it with the Creation Kit.
If the problem persists make an error report on the official DynDOLOD support forum.