In rcdc after load combination > add from analysis it is showing ....minimum one dead(vertical) load combination is required ... What to do
It suggests that your file does not contain a load combination which is only having gravity load case. I would suggest to create a new load combination with dead and live load only with factor 1. This additional load combination can be created in the RCDC load combination page also. If you right click there it will provide an option to add load combination.
Using that option you can add a combination consisting only vertical load.
this will solve the problem.