If you don't have the option to enable PayNow when you go to the invoice settings, then it could be one of two reasons, so you may need to check these:
1. If you are using a trial account - PayNow is not available for trail accounts.
2. If you do not have a Stripe integration, or your Stripe integration requires renewal (What is Stripe?)
Once you have an Active account and Stripe integration set up, you will be able to enable PayNow for your Formitize account.