If something is on an asset store or is a stock asset, it's fine to use (if it's a paid asset you need to have bought it). If it's public domain then it's all good. If it has a creative commons license then just follow the terms of the license and you should be good to go. Main thing is don't use anything copyrighted, stolen, or ripped from other games. Existing chunks of generic code not specifically made for your submission, like dialogue systems and player controllers, are fine to use.
With all that being said, it's encouraged to do as much as you can yourself and within the submission period if you're able.