Follow-up: So, I didn't actually watch the video that you referenced above. I just provided information based on what I know about custom:button-card and its templates. I decided to check out the video and see what the instructions were trying to tell you to do. My information is valid, but not in terms of that video.
I am a follower of Lewis but, personally, I don't recommend following his instruction from this video. I didn't watch the video completely, but looking through the comments, there seems to be something missing that actually makes everything work (perhaps lost in editing). There are plenty of questions in there just like yours.
Second reason: I don't recommend MinimalistUI, in general. It is very well made and obviously a lot of work has gone into it, but it can be difficult to work with and even more difficult when it comes time to make modifications to it to get something to show in a specific way that you want and not the way the Dev wanted. Additionally, unless you only want to run MinimalistUI, it becomes difficult to get other cards to "play nice" with it.
Now, with all that said, MinimalistUI is very nicely made and I do recommend using it as a reference to design your own cards. You can take the templates, and, following my instructions above, use them without actually having MinimaistUI installed. That's what I do. Kinda. I don't use the whole template but I pick out bits of code that I find useful. In fact, I noticed something new dealing with variables that is now being used that I'm going to look into a bit more to understand how (and why) the Dev used them.