This is caused by having a missing value in the custom field. It is possible that the value was accidentally deleted.
It is possible to simply readd the field into the custom field value list. A project admin or possibly workspace admin will need to make this modification.
It will be necessary to ensure that the definition is readded in the correct artifact type. By default the field edited will only be applicable to defects so it may be necessary to change the "type" and modify a user story field, for example.
Please see the following documentation for working with custom fields