When doing a pre-migration check or copying a site object, the following warning message is displayed:

The source column refers to a term that cannot be found in the 'My Term Set' term set. The column seems to be corrupted.
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This message is displayed when a term used by the source column is missing in your term set at the source.

Here is an example for a column named "Project" that is driven by a term set "Projects". The column Project is filtered to only show terms underneath the term "Construction Projects". Here is the term set:

In this example, the error would mean that the term "Construction Projects" was deleted after the column was configured, meaning that the column is now corrupted. Attempting to copy such a column will result in the above error in ShareGate Migrate.

Update the column's settings to either refer directly to the term set, or to an existing term.

In the above example, if Construction Projects no longer exists, the column could be fixed by making it reference either the Projects term set, or another term like Building Projects. This can be done by going in List Settings and clicking on the column, then choosing a term set or a term in the Term Set Settings section.