TL;DR

To use iMessage Text Message Forwarding on your iPhone and for Beeper to receive your SMS messages into Beeper, you may need to have Two-factor authentication enabled. You will also need to delete your iMessage connection in Beeper and try connecting again.

How to use iMessage Text Message Forwarding on Beeper if you only have your iPhone and no other Apple device?

With iMessage on Beeper, while having your SIM/eSIM on an iPhone, you can enable Text Message Forwarding to receive SMSs on Beeper, in addition to your iMessage chats.

If this is your case, having only an iPhone and no other Apple Device (such as another iPhone, iPad, or Mac), Apple's Two-factor authentication may need to be enabled. Otherwise, you might see a pop-up asking for a verification code when enabling Text Forwarding on the Beeper Mac.

You can confirm or enable Two-factor authentication on your Apple ID account on the Apple ID website.

• Sign in
• Go to Account Settings
• Confirm or enable Two-factor authentication

After this is done, if you have already attempted to setup iMessage using Beeper, you will need to delete the connection, and set it up again.