From Official Wiki of the Renaissance Kingdoms

You can find Taxes menu on your Character sheet, in the bottom, if you have taxes pending to be payed.

To see which taxes you're still owing, you select the option "You have taxes to pay!"

Taxes can derive from:

Each tax you have pending to pay is subject to a 10% penalty + 1% per day late. You have 7 days to pay the taxes without a penalty, and you can choose to pay each tax you owe individually.

You can also negotiate with your mayor or sheriff, since they have the option to cancel the penalties or the tax completely.