Person/Group Field finds a user, but won't add them when selected

Hey everyone,

I have a Sharepoint list and the first column is a Person/Group field. I can type the user's name and it finds their info. When I click the name to add to the field, it throws an error and says "You can't leave this blank". I am able to add other users to this field. Any suggestions?

Thank you in advance for your help!