freetobook connection

Description

Dear all,
freetobook software only requires your paypal email in order for the app to connect to your PayPal business account.
When a client tries to pay using their paypal account the error « we are unable to process your payment using your paypal account at this time. please go back to merchant or try a different payment method »
Happens for all customers currently
Am i missing a setting on the paypal setup that needs to be completed? or is this a freetobook issue?
Please help!
Thank You