sign into the Apple ID website and select Edit in the Payment & Shipping section. Make sure the info matches with your PayPal account there and save any changes you make. Try again once that’s complete.
If the issue continues, sign out of the iTunes Store on your device, restart, and sign back in. If you’re using an iOS device, go to Settings > Your Name > iTunes & App Stores, tap your Apple ID and sign out. If you’re on a Mac or PC, go to Account at the top of the screen or window and click Sign Out. Restart the device and sign back in. See if that helps.