First guess would be that the script you try to run has backtrader.py name. Rename it. If it doesn't fix the error, than you have some issue swith the installation. Feel free to google how to fix ModuleNotFoundError: error. Correctly installed and used bt doesn't return this error.
For code/output blocks: Use ``` (aka backtick or grave accent) in a single line before and after the block. See: http://commonmark.org/help/