Would it be okay to make my first game entirely out of store-bought assets?

it's up to you .. probably it would not be appropriate to see existing assets reused but if this does not bother you and satisfies you, leave it that way. Your choice is free. Instead, try to tweak them a bit at least to make the game visibly yours for you. I'm making a game, Check! lol We are happy to know and try your new game share it we are waiting for it

Absolutely, the first VR game I built and published on Oculus store used some store bought assets. I was in the same position as you are now, I didn't know how to build assets, over the years I have learnt how but its very time consuming if you are indie.

I still use store bought assets now, but in nearly every case I have to adjust to fit in some way. Be that aesthetic or efficiency reasons.

For your first I believe the most important thing is to pick a manageable game (i.e don't try building a MMORPG). Get through the entire process from conception to published. The fact you use store bought assets is irrelevant when learning.