Sandking4141 This is an issue that GrapheneOS users rarely experience. This is a Signal issue, so you might want to ask them or find a Signal community where they can give you suggestions.
I personally don't know what is causing it. Probably some anti-spam measure, which is closed source on their servers so nobody really knows how it is supposed to work. It could be that you're using a phone number that was previously registered to someone who was spamming others, or someone was spamming from the IP address you're using. It's hard to tell. Maybe someone here knows and will share their workaround.