If your customers get the error message "Invalid username or password" while logging into your online store, this can be due to 2 reasons:
Login details have been entered incorrectly
Check whether all information has been entered correctly. The password is case sensitive.
Checked out as guest
The other possibility is probably that they placed an order as a guest. At that moment a customer file is created in the back office, because orders must be linked to a customer, but no customer account is created. For this, customers must first register. This can be done as follows:
If customers create an account in your webshop with the same email address, the guest account is automatically converted to a registered customer. Previously placed orders are then linked to that account.
When customers click on "Forgot password" they will receive an email containing a password reset link. This allows them to set a new password and, in this case, create a password that they can then log in to. The guest account is automatically converted to a customer account.