Why can’t I order anything in apps?

Check your Restrictions on Settings. See here:

https://support.apple.com/en-us/HT204396

Check your Screentime restrictions:

Settings>ScreenTime>Content & Privacy Restrictions>Itunes&AppStore Purchases... the first 3 options must be set to "Allow".

Regards

Giulio

Are you receiving any error messages when trying to complete an in-app purchase?