The script can not be found.
Missing scripts are also reported in the papyrus logs as "Cannot open store for class X, missing file?".
The plugin/mod missing the script is most likely not going to work as intended.
The Unofficial Skyrim Special Edition Patch fixes missing vanilla scripts.
This can happen in case not all required *.BSA files are not loaded for whatever reason. The log prints a list of every *.BSA file that is loaded. Check that none of the required *.BSA files are missing by comparing the log messages to the load order.
This can happen if a mod is installed that requires the scripts from another mod that is not installed. Verify the requirements of the related plugins or mods.
This can happen if not all required scripts where included in the download archive of a mod or if there are typos in records defining the filename of scripts.
The log tries to only report missing scripts in case they might actually be used in the game. Mods sometimes contain orphaned or unused records or data, which is not causing problems directly.
If it has been verified that the script is not required try to remove or update the unused record/entry so the non existing filename is not being referenced.
In case a script is missing from a mod archive or installation, notify the mod author.