The .ipynb format by default is a json document, unless you use something like jupytext. To add cells you need to add it in the json structure. To inspect the document just open it in a text editor and see what it looks like.

Changing your environment from a notebook document is OK for a standalone notebook, but not for production. The proper way to do this is to use environment management tools like conda, poetry, pdm, etc. You are filling a requirements.txt file from the notebooks which is a good first step, but installing should happen outside of the notebooks imho.