Someone sent me money and I am unable to receive it via Apple Pay.

You will need to make sure you have Apple Cash set up. You will have 7 days to accept the money, but that can only be done with Apple Cash where the money will be deposited. There are eligibility requirements to use Apple Cash. If you do not accept the money within 7 days, it will automatically be sent back to the sender.

Set up Apple Cash - Apple Support

Apple Cash is a service that’s administered by Green Dot Bank. Contact an Apple Cash Specialist at Green Dot Bank by calling (877)-233-8552 or by calling 1-800-MY-APPLE

https://applecash.greendot.com/termsconditions/