I'm afraid that if I don't see the code nor the errors you're facing I don't know what you're struggling with. If you don't have the need for modules at first, try sticking with just one central Vuex store and continue from there when the need arises.
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I think the official documentation is quite simple, have you checked this out: vuex.vuejs.org/guide/modules.html
I'm afraid that if I don't see the code nor the errors you're facing I don't know what you're struggling with. If you don't have the need for modules at first, try sticking with just one central Vuex store and continue from there when the need arises.