Somehow I managed to replicate the error!

replicated the error

I believe you have created eks cluster using some other AWS user.

and you are trying to access using admin user, which means by default you can't archive that. Because initially eks cluster owner can able to access the eks cluster and he can only able to give access to rest of the user.

If you need to access eks cluster using your admin user, it means

Go to eks cluster console > click on to your cluster > access

enter into eks cluster

then create an access entry

navigate to access section

and select your respective user and make type as standard, click next then select amazoneksclusteradminpolicy and access scope as cluster then press add policy

create access for your user

now you can able to access the cluster.