Custom fields enable the user to create his own user-defined fields that would be embedded later in different work items using Field Layout settings.
In the event you have reached the system limitation, it is recommended to review the use of your existing custom fields and free the ones that are not used.
The system may give you the following warning “You have reached the maximum of allowed <FieldType> custom field”
In this case, review and remove the unused custom fields to allow to define a new one.
From the custom field, summary view (Admin -> Custom Field) Look at the work item column and make sure it is not being used by any work item.
While deleting, in case the custom field is not being used by any work item, the system will delete the custom field directly
In case the custom field is being used in any of the work item, the system would display the confirmation message before the deletion completes
Now after removing unused customer field, try to add new one.