You’ll probably need to provide more details about the problem. NISEI OP supported formats is the correct place to look.
The problem most likely is with specific decks on NRDB as they are published by individual players, possibly not using the most recent legal set. For example, you can see that Stimhack is in the original core set, revised core set, and system core 2019. Only one of those sets is “legal” in standard and having any Stimhack in your deck still makes it Standard legal. But, if the creator of the deck is using the original core set Stimhack, it will say that it’s not legal. NRDB is not smart enough yet to ignore cards from older sets if they are in newer sets, but does have a way for deck creators to automatically change to the most recent version of cards that are part of multiple sets, but not every creator does that every time.
It could also be that a deck is using all legal sets, but has a banned card, so you also need to review the cards in the deck against the banned list.
Does that help figure out the problem? If not, with what specific deck or card do you see a problem?