A known issue exists when adding Managed Google Play Store apps for users to Mobile Device Management in Intune. While the process is described here in detail, the step in which admins select the application seemingly does not allow them to click on Select and prevents them from continuing.

As reported by other users on the internet, you should click on Select regardless and not be alarmed by the platform notifying you that your choice is confirmed. The next step is to click Sync and wait for the app to appear under Android Apps view.

If you still have problems, please don't hesitate to contact Microsoft Support for advice.