However, what are some common mistakes beginner real estate investors/Rehab flippers make?

I’d also love any recommendations for beginner-friendly books, podcasts, or other resources that are suitable for a teenager. 

For beginners, focus on markets with lower entry costs, high investor activity, and steady demand—like parts of the Midwest or the Southeast.

It's not the reality for most and if more beginner investors had the correct mindset they would make better and more informed investment decisions.

The course heavily relies on cold calling and emailing, which hasn't been effective for me as a novice.I'm considering a different approach, such as arbitrage, as it seems more accessible for a beginner.

Its simple arithrimetic but often there is an angle and no prop is the same.  6)  good beginner rule I use for myself is that for a buy and hold multi - if it needs more than 10k of work its probably not a great FIRST investment.  

Beginners Luck! 

Both strategies offer valuable lessons, but the last thing you want is for a beginner to overleverage themselves and take on more than they can handle.

Avoiding Mistakes: What’s the one thing you wish you could go back and tell your beginner self?

Start with BiggerPockets Ultimate Beginners Guide (free).