I can't create the group 08.11.11.207

Hello 

When I want create the group, Ps give me the info "Select main part of group:
* WARNING UNIT NAMED 5723D ALREADY EXISTS - INPUT REJECTED.
Select part of a group:Cant retrieve Group from selected Part"  Version 08.11.11.207 Acad 2016 Win 10

Normal beam and plate.

What is it?