Getting the insufficient permission issue while trying to login into developers.ringcentral portal

Your developer account credentials are different from your sandbox account user credentials.

If your developer account is created automatically for your production account, you need to enable a user in your production account to get the permission to access the developer portal via a role. See attachment.

If your developer account is a free-tier account, check you email to see if you still can find the email when you signed up. Or give me the email address you used to sign up so I can check.