The column which you are setting as the alternate key, has that already got an index defined?  If so, remove the index and repeat the process of setting the alternate key.

None of the columns are indexed, this is what has me stumped.

Have you got conflicting data i.e. duplicate records against the alternate key column.  If so, you will need to clean up the data first as well before setting the alternate key.

There is a similar thread “Custom Form Primary Key” that highlights a similar if not the same issue.

I will look into this, thank you for responding!  This has given me hope!