After purchasing and installing a new Acronis license in Plesk, Acronis Backup extension shows License Activation page and requires credentials.
A brand new license is not associated with any Acronis account yet.
Create an account by filling out the form in Acronis Backup extension > License Activation page > Create a new account. Otherwise, use the existing account to log in under I have an account login form.
Refer to the Acronis Backup extension documentation for more details.