First, it's possible that the version of the mobile app is outdated. Please tap on the Google Play app and install the latest version.
Next, you can also force stop the app and clear the app's data & cache. Go to Settings > Apps & notifications (if a list of apps is not displayed, tap on "All apps" or "App info"). Then, tap on the app that is crashing and tap "Force stop." Now, tap "Clear data" and "Clear cache" and wait for the system to process these actions. Finally, reboot your phone and use it as you would normally.
Note: if specific apps continue to crash, even after rebooting your phone, you can reset app preferences to go back to default app settings (note: before you do this, please note that this process resets all apps back to their default settings). Go to Settings > Apps & notifications > All apps. On the "App info" screen, tap on the menu icon (three dots) in the upper-right, then tap "Reset app preferences."