My client is trying to enter his Zoho credentials for the portal but it is not going through. What’s the issue?

No, your client’s zoho credentials are not to be used in order to log into the customer portal.

When you send a portal invite to the email address of your client and they accept it, they will be requested to create a new password using which they can log into the portal you’ve invited them to. Their Zoho credentials has no relevance to this, and even clients without a Zoho account can log into your portal using their email ID and the password they created.