We have different versions of our website to offer products in different currencies, so one reason why you may not be able to login is that you're on a different version of the site to where the account was first created. For example you're looking to purchase in USD but have created an account via our UK site.
A way to determine if this is the cause of the issue is by sending a password reset request. If you don't receive the reset email, you are probably on the incorrect version of the site or there's a typo in your email address.
If you're still having issues logging in, please get in touch with our team.