When doing a pre-migration check or copying a site object, the following message is displayed:
The term set 'Missing Term Set' could not be found at the destination.
The message is displayed when the term set used by a source column is missing at the destination (because it was not successfully migrated). For example, you have a column that is driven by a term set called "My Term Set", this error means that "My Term Set" was not migrated successfully.
Migrate the term set from your source to your destination. In the above example, "My Term Set" should be migrated to the destination. You can find more information about how to migrate term sets with Sharegate here.