|This error will be returned when trying to create a lookup field on Activities to an object that is already connected through an existing lookup field on Activities. Each custom lookup field can reference one type of record, and no two fields can look up to the same type of record.
Please be aware that the feature to create Custom Lookup Fields on Activities is still in Beta. It is production quality, but has known limitations. You can provide feedback and suggestions for activity custom lookup fields on the IdeaExchange.