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I would contact the developer of the extension you are referring to directly and ask them if it's compatible or if they are aware of the possible misbehavior you are seeing.
I updated my answer to include refreshing the cache. Are you still having issues after completing those steps? If so, then there is something else going on. Start by looking at the log files in the var/log directory. And if you possibly have another module that you were developing still enabled, don't forget to disable/remove it.
I'm not sure what you mean when you said "but that is not logged in". Did you clear the cache afterwards? I'll update my answer to reflect having to clear the cache.
Best place to start is by reading the Magento User Guide. It's thorough and will take some time to read and understand everything, but don't sell yourself short by just skimming it. It sounds like this video game store is your livelihood so invest the time in yourself sooner than later. Magento also offers courses through Magento U. They can be pricey but nothing beats investing in yourself and I would argue no one knows Magento 2 better than Magento.
