Could not find server 'server name' in sys.servers. SQL Server 2014

I had the problem due to an extra space in the name of the linked server. "SERVER1, 1234" instead of "SERVER1,1234"

At first check out that your linked server is in the list by this query

select name from sys.servers

If it not exists then try to add to the linked server

EXEC sp_addlinkedserver @server = 'SERVER_NAME' --or may be server ip address

After that login to that linked server by

EXEC sp_addlinkedsrvlogin 'SERVER_NAME'
                         ,'false'
                         ,NULL
                         ,'USER_NAME'
                         ,'PASSWORD'

Then you can do whatever you want ,treat it like your local server

exec [SERVER_NAME].[DATABASE_NAME].dbo.SP_NAME @sample_parameter

Finally you can drop that server from linked server list by

sp_dropserver 'SERVER_NAME', 'droplogins'

If it will help you then please upvote.

I figured out the issue. The linked server was created correctly. However, after the server was upgraded and switched the server name in sys.servers still had the old server name.

I had to drop the old server name and add the new server name to sys.servers on the new server

sp_dropserver 'Server_A'
GO
sp_addserver  'Server',local
GO