The message indicates that you have set the 'required' property to true on the reference selector. This is not needed if you have an validation rule in your domain model. That rule will be applied, regardless whether the required property is set on the reference selector.
Don't know about your specific situation but it is not possible to specifically set a validation rule for a reference selector in the domain model. One way around is to use validation feedback microflow activity.
Didn't see the other answer... Reaction on Michel: This is not the behaviour I experience. The validation rule is applied when I enter new 'projects', but when I use 'project' as a reference selector (either nummer or name) the rule is not applied. Hence, I don't see the error message. However, a record is inserted leaving the project reference empty.
Reaction on Karol: Yes, microflow could be done, but I think this could (should?) be modelled very well in the domain model.