SSMS Permission Denied, Login with Windows Authentication
You are just a member of BUILTIN/Users when connecting to SQL Server, your USER\Administrator is not a member of sysadmin role, so you have no server permissions. Try to execute

select * 
from sys.fn_my_permissions(null, 'server');


and all you see will be CONNECT SQL + VIEW ANY DATABASE.
Of course you cannot use DAC, you are NOT sysadmin to be able to do it.

What you should do instead is launch your server in single user mode, and as you are local admin, you'll be able to log in as sysadmin.
After that you should add yourself to sysadmin role.

Here are the steps to perform: Connect to SQL Server When System Administrators Are Locked Out