The PayPal account's payment source isn't set to immediate funding.

Make sure the payment source in your PayPal account is set to "immediate funding source." For more information on PayPal payments, contact PayPal support.

The PayPal account wasn't connected.

Some browser settings interfere with the save button when you add PayPal as a payment method. If you have trouble trying to save your PayPal information, disable all pop-up blockers, browser extensions, and plugins, and save the account information again.