If your employee cannot log in to your business account, make sure they have first created their own account (a customer account is sufficient). They can register an account here. Only after that can you set up their access in your business account. This is a common problem when employees are not registered properly, leading to access issues.
ATTENTION: If you set up access before the employee created their account, you will need to uncheck their access in the staff section settings, then set it up again. This will send a new invitation, allowing them to set a password and log in successfully.
For further guidance on setting up employee access, you can take a look at this article.