PayPal to Mpesa error "Sorry, request could not be processed" means that you need to verify your profile.
Upload your bank statement, National ID and KRA pin. Ensure that you have edited your PayPal or KRA address to match. If asked for transaction history, provide the transaction reference numbers.
You can upload the documents via dashboard in the Remove Limit Section or Edit your profile via settings or through this link https://www.paypal.com/policy/hub/cip.
The method to use depends on the option available in your account.