Do you have other gacha mods or the original gacha club installed?
The problem is Gacha club, the packages of Gacha Club and the Gacha luminal app have the same name(You can see the name of the package with a file manager such as Zarchiver, etc), which makes your device think that Gacha club and Gacha luminal are either, the same app, or that the package of gacha luminal is invalid due to having the same name as another package.
You will have to uninstall Gacha club to be able to install Gacha luminal, however, if you do not want to uninstall Gacha club, you can solve this issue by changing the name of the package with an Apk editor, your device will no longer say that the package of Gacha luminal is invalid and you will not need to uninstall Gacha club, this method is not difficult to do once you understand how to do it and it will solve the issue of every Gacha mod that gives you this error(Invalid Package or asking if you want to update it instead of Install it)
I hope that my words weren't too clumsy and that you could understand what I meant 🙏
Apktool M is easy to use, just change the name of the package a bit(replace or add a letter) and it's done, like i said before, this doesn't apply to only Gacha Luminal, it should solve the issue of any Gacha club mod that gives the message "package is invalid" and "do you want to install an update to this existing application?" when trying to install it.
It shouldn't take too long to do this, let me know if it worked.