So the problem is that something has gone wrong either in your ssh config or in the way ssh is resolving the host that your are passing. You can clearly see in your screenshot that ssh is tryng to connect with username "sharonmathew" and not with your ptfs account.
Update your configuration to this:
Host csnhr
HostName csnhr.nhr.fau.de
User ptfs268h
IdentityFile ~\.ssh\ssh_key
IdentitiesOnly yes
PasswordAuthentication no
PreferredAuthentications publickey
ForwardX11 no
ForwardX11Trusted no
Then attempt to connect with
ssh csnhr
if it still does not work please make sure that
A: ~/.ssh/ contains a "config" file
B: That the contenents of the "config file" are actually what you posted.
C: ~/.ssh/ actually contains a file called ssh_key (wich actually is the counterpart of what you uploaded to the HPC portal)
In general is a good start to delete all the stuff you put in your config file and just try to make one Host configuration work first since you may "implement" ambiguity.