Need more details. What is the source? Geodatabase? SQL? If so, what flavor of SQL? You cannot change the schema of a feature service, only the data used to publish the service. However, I see that error all the time when there is something going on in SQL that is unsupported by Pro (e.g 2 or more left joins on a view).
PostGres. Feature Service from an ArcGIS Server 10.7. This schema is unlocked as shown in screenshot and I'm fairly certain I've added/deleted fields from it before. However, all I want is to be able to open the design view so as to be able to turn on/off fields for easier viewing.