Hi, usually the error 10548 is related to the fact that during the checkout process your module is calling a method that is not allowed for your PayPal configuration, the method is DoDirectPayment and allows merchants to take card payment's detail directly from their web site. This method is only allowed for customers in UK, US and Canada that enabled Website Payments Pro.
If your PayPal account is already upgraded to PRO then I suggest first to uninstall and re-install the module you are using and if reoccurs then open a request to MTS so that we can investigate the root cause.
Open a ticket with us by loading this link MTS and at the bottom click on "Contact Support"
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