Everyone has gone through this at some level, some more than others. Personally, I did a ton of Udemy courses and when I got done I pretty regularly hit a roadblock.
First, don’t get down on yourself or freak out. Again this happens to the best of us. You see I learned (far too slowly) that it is far more viable to learn how to read docs and then use what you learn in a project instead of a course.
OR if you like courses, as I do, then go ahead with the videos and material so that you hit roadblocks and challenges. That if you haven’t taken the hint is the key to learning a new programming language or framework.
Now while it is simple to say that using Var post ES6 is simple, and technically correct, why would you not want to learn about the why? They why allows you to see the bigger picture. Additionally, you will likely see code examples and legacy projects that use this code. You need to understand it because if you do not, and you do not fully grasp the differences between let and const then you cannot update the code from a past style of code to a current best practice.
I digress. The point is this, as software engineers, our job is to learn as we do our job. Always be learning, and that means always be playing and experimenting with different coding languages, frameworks, libraries, etc. If you do not take the time to hammer out code then you will not create the muscle memory. That muscle memory is how you will hone and sharpen your skills. If you want to be successful you will have to hit those roadblocks, and hit those deadends.
Take the time to fail a few times, because learning from that failure will make you a better software engineer.