4 possible reasons for the error:

1. Your friend has not actually met the requirement for said duty. Please verify that he/she has and is able to queue for it and meets the level/ilvl required.
2. Your party composition is not allowed, i.e. unsynced or 2 tanks in 4 man, etc.
3. You did not end the party recruitment. Open the recruitment and at the bottom, there is a button to cancel recruitment. Click that and hit accept. Verify by seeing that your party is no longer listed.
4. The server instance is having congestion problems and is failing to load an instance for you.

Hope that you get to jump in with your friend soon.