Hi I bought a new mackbook pro 13 2015 running on seirra ox, but I cannot install apps, it asks me administrator name and password, I HV already made a apple ID and tried using it but doesn't work. Also I HV the password which we use when the mac boots up,. I HV also seen the name of previous owner is still there.
Since you bought the system used, contact the prior owner for the administrator password. That system should have been wiped clean like a new system when you got it, it is a really bad idea to sell a computer with the old account on it.