Admin cannot masquerade in as a learner.
Is the learner an SSO user? If so, you will most likely need to use a non-SSO account to login and masquerade. This is because two SSO accounts will conflict with masquerading.
Make a 2nd(non-SSO) account for yourself and make your account a site admin:
Logout and log in as that new non-sso account at /user/login2
You will then be able to masquerade as any user.