When migrating data from a list that has record declaration enabled or the list itself, you get the following message:

Unable to reset the original record declaration settings of the list. ShareGate Migrate cannot continue since this may cause undesired behavior.

ShareGate Migrate cannot create the record declaration fields since they are built-in. Even if record declaration is activated on a list, it might be missing fields. To fix this, ShareGate Migrate will toggle record declaration settings which will trigger the creation of those fields. This error happens when ShareGate Migrate has started the toggling process, but when trying to reset the settings to the original settings, an error occurs that keeps ShareGate Migrate from doing that.

Go in the declaration settings of the destination list, check or uncheck the 'Automatically declare items as records when they are added to this list.' checkbox, save and then do the reverse operation. This will create all necessary fields. You can then migrate again and the error should not appear.