Hi 18-06,
From the information you provided, it sounds like Screen Time is enabled and there have been restrictions placed on this device. To view information about restrictions, click here: Use parental controls on your child's iPhone, iPad, and iPod touch.
In the article, you can see the different type of restrictions that are available. The restriction you have may is in Settings > Screen Time > Content & Privacy Restrictions >Allow Apps.
If the restrictions were set up by someone other than yourself, you may receive a password prompt. The password is needed to make any changes. You may also see a password prompt if you set up Screen Time yourself and added a password.
Best wishes.