Working on my own application and in need of some help.

As an entrepreneur I am always looking to improve the way I do my job.
As a bookkeeper, my work revolves around collecting and processing information.
In the past I have already used various applications to do this.
None of the applications worked sufficiently for my liking.
I am currently working with a no-code application from Triggre.com, but this one is also not sufficient.
So I want to build an application for my own use.
After an introductory course for Python, I am now learning about Flask.
I learn about Flask through Miguel Grinberg's book.
Once I'm done with this book, I'm going to delve into HTML, CSS, Javascript, and SQL.
I came to this community (now) because I got stuck on an error message.