The single biggest mistake founders make is building but not marketing.
Sometimes it comes down to a lack of knowledge but sometimes it's operating under the mistaken belief that "a great product will sell itself".
Yes, in some cases a great (or good) product will go viral without much need for marketing spend.
The audio-chat app Clubhouse (remember them?) exploded in popularity during 2021 due to exclusivity (it was invite-only) and the participation of high-profile celebrities and business leaders.
But they were an outlier and an exception to the rule.
You are probably not an exception to the rule, to be honest.
So if you're building a product (or have plans to), make sure you know how to create awareness in the market for what you're selling, and how to monetize that attention.
Otherwise you're just someone with a product, not a business.