Hi,
At the moment, there is no Required property of a control in PowerApps. There is Visible or DisplayMode which you can utilize.
Supposed you have a checkbox, and based on the state of this checkbox, you show/hide a warning message to a particular label(s) or any control.

In my sample image above, if CB1 is checked, the warning message for CB2 will show. The formula is applied to the Visible property of the warning message label of CB2.
NOTE: this does not validate CB2 on form submit. You have to do CB2 value validation separately.
Cheers.