Why is «Enable this Solution» backend control disabled for the «PayPal Payments Standard» payment method?

• Fill the credentials fields:
  • «Email Associated with PayPal Merchant Account»
  • «API Username»
  • «API Password»
  • «API Signature»
• Disable other PayPal payment methods («PayPal Express Checkout», etc.)
• Disable «Enable PayPal through Braintree» option for the Braintree and BraintreeTwo payment modules

See also:

• Why is «Enable this Solution» backend control disabled for the Braintree payment module?
• Why is «Enable this Solution» backend control disabled for the «PayPal Express Checkout» payment method?
• Why is «Enable this Solution» backend control disabled for the «PayPal Payments Standard» payment method?
• What does <requires> tag mean in a backend config field XML declaration?