I just checked that on PC and I confirm that their paid service is not working.
I submitted several transaction IDs with the fee rates ranging from 8 sat/vbyte to 15 sat/vbyte to their paid service and got error for all of them. It only displays a message at top of the screen saying "Error" without any explanation on what causing the error.
It appears that this paid service was simply added as a design feature or it might be temporarily disabled due to some reason.
I can't find any announcement on Viabtc about this there might be a reason why it does not work.
Do you have any balance or deposit coins to Viabtc?