Face recognition "don't show again" error - unable to undo ?

Without knowing more information about your situation, I don't think I can be more helpful to you.  How many total photos do you have in your catalog? How are your photos named and organized in folders? How many photos have been tagged?  Did the person whose stack was deleted usually appear alone? Or do they usually appear with another person or persons?  If the latter, then filter the grid by the name of the other person and visually search those photos for the name of the don't show again person.  Either use the Mark Face tool to add the person's name to the photo or simply drag and drop that person's People tag onto the photo. Adding just the tag without marking the face will result in a generic icon being placed in the People room instead of their face.  But at least if you search via the tag, the person's photos will appear in the results.

If you know which photos were in the stack, try deleting them from the catalog and import them again.  (If this doesn't work, I have another suggestion.)

Alternatively, you can use the Mark Face tool to add the People tags manually.

It's in Photoshop organiser in Elements 2021

Hi, please tell us the exact application (lightroom classic or CC?) that you are using, as it is not Photoshop...