Hey there!

This error means the app you want to install is already installed and the installed version is not signed the same as the one you want to install. To work around this uninstall the app first then try again.

You may need to uninstall it from all accounts on your headset.

It is recommended that you uninstall it from the desktop SideQuest app if you don't see it in unknown sources. You can do that in the my apps screen on the SideQuest app where you can find the app using its name or package name ( like net.kdt.pojavlaunch for QuestCraft ). Click the cog wheel icon then on uninstall.

God speed traveller!