Hi Giuseppe,
Thank you for reaching out to us. I am apologizes for this inconvenience. Based on the error message, it looks like there may be an issue with how discounts are being calculated and passed to PayPal, causing a mismatch in the total amount validation.
To investigate further, we would need temporary admin access to your backend. Could you please provide us with the necessary credentials in the hidden content section?
Additionally, before we proceed with troubleshooting, we strongly recommend that you take a full backup of your store to ensure no data is lost during the investigation.
Looking forward to your response.
Best regards,