I had problem in capistrano installation : if I connect to the server from root and then switch to jenkins user ( su jenkins ) and I run this command (cap ) capistrano work just fin , but if I connect directly to jenkins user capistrano never work even when I already installed .
First check your capistrano installation location and Jenkins pointing location are same or not, seems to be Jenkins user is not having permission to run the capistrano, When you are running without Jenkins you are giving su Jenkins command to switch the user, try without su command, if it is not running as expected you can consider it is a permission issue , then add Jenkins user into same admin group. then it will work.