I have two Windows server, and on each has SQL Server installed.
One server calls SQL1, and the other SQL2.
I was able to connect to SQL through any server, but when I try to connect in SQL1 to the SQL2 instance, it does not work.
The error is telling you what the problem is - you are logged into the server as the cluster admin which does not have access to SQL Server (and should not have access).
Are you trying to setup a cluster? If so - are you trying to setup and configure an FCI (failover cluster instance) or setting up an availability group?