Why can't I login to the Admin?

This can have three causes.

Firstly, you need to make sure that you have a username and password.

Secondly, you need to make sure that you are using the correct username and password. Are you copying the password from an e-mail? Make sure that you select the password correctly, without spaces before or after.

Was your account created by another administrator? Ask him/her to check if you are assigned to the correct role that has access to the Admin.