Hi,
and welcome to learning that everything is in development and have fun finding all the human imperfections 
Unreal has a lot of pre setup systems that you can easily break without realising and creating a bunch of errors. Many of them probably won’t be useful to you, just because it’s such a wide range of tools.
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Lighting: Did you bake the lighting? There are different ways to approach lighting, but I think the default way still is to precalculate it into static light maps (textures) and lightprobes. I think Unreal doesn’t bakes them automatically when you package/build your project.
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Nanite: Is supposed to be a better use case for highly detailed photogrammetry models. But it is a bit of a mixed bag right now.
There are people saying, that you need to model things like foliage with no masked materials to utilize it corretly. https://youtu.be/Vzz8_O3PIUg?t=193
Otheres are more say taht it intruduces more problems than it is solving. https://youtu.be/M00DGjAP-mU -
Packaging: Best is to search for warning messages or parts of it, like an error code. I also suggest to test bigger changes of your project in a fully shipped build.
I once had 3 different outcomes for a single object from Play In Editor (PIE), Standalone and Shipping Build. You very well may never encounter such a problem, but that one time made me question the process
I would recomend looking into the starter content. You need to download these (a few gigabytes) in FAB as the doc states (don’t know it myself anymore). There are maps/levels (the orange marked assets) that showcase functionalities.
Also look into how Blueprint Interfaces work. They are very handy in creating modular interaction concepts. You basically create functions that can be attached to any object and the object decides what to do when one of the functions is called. https://youtu.be/wGmlKbllPcw