If you can't log in to BRM it will probably be for one of the following reasons
Wrong Data Centre?
You are accessing the wrong servers, we have data centres in the USA (US) and Europe (EU)
If you are using the legacy login, you need to choose the right one:
But if you are using the new smart login (only BRM2.0) it will automatically redirect you to the right data centre.
Wrong email address?
Perhaps you have two email addresses or more. You can only log into the one which has been invited to BRM
You don't have an account yet?
Perhaps you don't have an account at all yet on BRM.
To get an account either start a BRM free trial
Or ask your store admin to invite you to the store.