The most common reason that you may not be able to log in to your Reviews.io account is that you may be using an incorrect login URL.
UK Users
If you are in the UK then your account is likely to be on the Reviews.co.uk domain, and therefore you should use this login URL
USA, Australia and Rest of World users
If you are in the USA, Australia or any other non-UK location then your account is likely to be on the Reviews.io domain, and therefore you should use this login URL
Forgotten password
If you have forgotten your password the you can reset it from the "Forgotten password" link on the correct domain for your account above.