The city center of Maastricht prioritizes cyclists and pedestrians and therefore there are only few parking spots on the streets. You will have to buy a ticket anywhere in the city center from 08.00 to 18.00 (on Thursdays the time slot is extended until 21.00). The rule of thumb here is that the closer to the city centre you park, the more you can expect to pay.
However, Maastricht has several (cheaper) Park + Walk and Park + Ride locations close to the city center. Visit Maastricht Bereikbaar has a handy overview of the best places to park. The site also contains a lot of information about different zones and parking spots in town.