Welcome, Dear Evgeni,
Thanks for reaching PayTabs out on our Developers Community. My Name is Osama El-Hamahmy the technical support team and I will be looking into this post and following up with you.
Regarding the mentioned error message "Authentication failed" which means that the PayTabs system failed to authenticate/identify the sent request details from your WooCommerce plugin side, therefore you have to check your account/profile details endpoint region, Profile ID, Server Key, and Client Key to find your account/profile details please check our solution articles:
How to get your account information from PT2 Dashboard?
How to get my Authentication/Integration/API Keys?
I hope this resolves any confusion or inquiries you may have,
You should also know that we are providing you with a great "Solutions Portal" that includes so many articles that can walk you through any situation such as the "The How to & The What Is (FAQ)" category which contains the technical and non-technical FAQ, and the "How to Integrate with PayTabs?" category which contains all the available methods to integrate with the fully detailed documentation and "Troubleshooting Your Issues/Errors" category which will allow you to trace and solve your issues without seeking for any assistance.
Please don't ever hesitate to comment back if you still have further inquiries. Let me know if there's anything else I can help with.
Glad to be always in help.
Hoping you have a profitable and successful trade.
I need your help, after I have installed the wooCommerce plugin this message displays on the checkout page!