There may be several reasons for this: if the monthly limit for card transactions has been exceeded, if the wrong PIN has been entered, or if some options are enabled on the card settings to prevent a transaction. Another reason could be that the merchant does not accept MasterCard, but only Maestro or vice versa.
First of all check the card settings in the application by going to the "Finances" section, select the card, click on "Settings" and check if all the necessary options are enabled.
If the monthly limit for card transactions has been exceeded you can change it yourself in the application:
Finances -> Select card -> Settings -> Card Limit -> Monthly Limit -> Edit