My customer is trying to log in to the customer portal using their Zoho credentials but they are not able to log in. What’s the issue?

Your customers cannot use their Zoho credentials to log in to the customer portal.

When you send a portal invite to your customer and they accept it, they will be requested to create a new password, which they can then use to log in to the portal. Their Zoho credentials are irrelevant in this process. Even customers without a Zoho account can log in to your portal using their email ID and the password they created.