How to resolve TF20015: The field {Field Name} contains the value {Value} that is not in the list of supported values

I got a call from one of my customers that when a user is trying to open work item he get the error-

From looking inside the template of the work item I saw that the filed have a rule of VALIDUSER, from further investigation I found that the user was renamed from Cathy Smith –> Cathy Jones.

Each team the work item was opened the value “Cathy Smith” was thrown as an error since the user “Cathy Smith” didn’t exist anymore.

The solution to the problem was to add to the field the rule ALLOWEXISTINGVALUE, This rule allows a previously entered value to still be valid even if that value is no longer a valid value.