transaction rejected by supplier (e.g. invalid mobile number) (error code 81)
The used mobile number is either invalid/unknown or not active in the carrier network.
Please verify that you have chosen the correct product for the SIM-card (correct mobile network operator?)
Also please check your entered number for any kind of typos.
Please note that freshly activated SIM cards usually need to wait a few hours before they can be topped up.
If this error persists, please note down the error code and get in touch with your Mobile Network Operator.