Invalid as in on the activation website? If so, try again after 50 minutes if you previously started an activation attempt but clicked on "back" at any point.
Did you pass the payment section?
If yes check if your card was charged.
If it was charged put your sim card in your phone it will likely work.
If it does work and you were charged go to link below to create your self serve account.
If you were not charged. System locks sim for 50 minutes. Wait one hour, clear internet cookie/history cache open a fresh browser in incongnito mode and try again.
Don't forget a referral code for a $10 sign up bonus. If you used one and were charged and service works the credit will be applied within 72 hours.
Welcome to Public Mobile 🙂
It’s says it when I’m trying to activate the SIM card... tried it many times in different browsers
If you've waited 50 minutes since the last attempt, cleared your browsers caches/restarted your computer, and the activation website stilll states "invalid sim", this means that either that the number has been mistyped or that the sim card has been activated before.