Authorize.net PayPal is not enabled
Starting Authorize.net PayPal Express (createAuthorizeNetPayPalExpressCheckout / start route) failed because isAuthorizeNetDigitalPayPalEnabled() is false (503).
Why this happens
Authorize.net card checkout may work while the PayPal via Authorize.net digital wallet option is still off or incomplete. Distinct from standalone PayPal gateway errors.
Diagnose and fix
1. Admin: enable Authorize.net PayPal / digital wallets under Payment; save related credentials.
2. Confirm base Authorize.net is configured first.
3. Member: use card checkout or native PayPal if those are enabled instead.
4. See Authorize.net and PayPal for the two different integrations.