Hi noukynouky,
I understand that you'd like to offer an iBook that you've created on the iBook store, but attempting to log into iTunes Producer results in an error stating that the Apple ID and password combination that you've provided is invalid. Verifying that your Apple ID and password work correctly on our
Apple ID
page was certainly a good call. However, this error is due to your Apple ID not being enabled for iTunes Connect.
See the information outlined below for steps to get set up to sell books on the iTunes Store.
Offering your books on the iBooks Store is free. Additional details on iBooks Store sales terms are available during the sign‑up process.
To offer your books on the iBooks Store, enable your Apple ID for iTunes Connect, provide your publisher name, and indicate whether you will be submitting your own books or submitting books on behalf of a company. You will then be sent an email with a link to verify your email address and enable your iTunes Connect account.
More information regarding countries that you can offer your iBook in and the signup process can be found at the link below.
Working with iTunes - Authors & Book Publishers: Frequently Asked Questions
Additionally, see this link for help with iTunes Connect:
iTunes Connect Resources and Help
All my best!