Messages on MacBook isn't sending or receiving messages, despite being signed in to iCloud
The first step would be to make sure you can send iMessages to other iCloud users. If not, then you’ll want to Sign Out of iCloud under Messages → Preferences → Accounts. You may need to reboot your MacBook before signing back in.

If it’s regular text messages (SMS/MMS) that aren’t working, on your iPhone open Settings → Messages → Text Message Forwarding and make sure your MacBook is enabled. If it already is, toggle it off, then back on. You will be prompted to enter a verification code sent to your MacBook.

If none of this works, you may need to reboot both devices with everything disabled, then reactivate everything only after booting back up.