RE: Dynamics 365 Web portal
Do you see a Portals tab in your Site Map?
If you go to Web Site Bindings, what is the value that you see in the Site Name?
Copy the Site Name and go to https://{Site Name}
Is that what you are doing? Does the portal appear?
What happens when you click on Sign In button? Do you see Sign in Page?
How are you trying to sign in? Go to the Contact record that you want to use for sign in.
Change the form to CONTACT: PORTAL CONTACT, check what the User Name is for the Contact. Click Change Password, and set the password for that Contact. Sign in with that Username and Password. If no Username, in the Portal select Register.
Make sure that for your Contact, there is an Associated Web Role for Administrators (if you need to Administer the Portal).
If you are still getting an error, can you share a screenshot of the error that you are getting if you are getting one?
Hope this helps...