The best way to convince someone that you can do something is to actually do it. Instead of telling people, show people. Put the proverbial proof in the pudding.
If you want to convince investors that you can get traction, get traction. If you want to prove to a potential employer that you do a given job, started doing the job before it’s yours.
It might mean working for free to prove yourself. It might mean preemptively offering introductions. It might mean preemptively providing advice or ideas. It might mean displaying your past work or even writing blog posts that show you have the right knowledge.
Lack of credentials or past results are easier to overlook when you can prove your abilities through delivering results. Results always speak louder than words.