Hi @AliceKoh - welcome to Streamlit!
I’ve read through this thread and it’s difficult to see what your problem is exactly. On the one hand you’re talking about Jupyter and on the other hand saying you wish to run that app in Streamlit. The two platforms are quite different and apps for each one are built differently. Also the code snippets you have shown are incomplete / incorrect. Anyhow, I figured you’re building a BMI calculator and since I’m trying to lose weight 
I thought I’d experiment with your code to make it work in Streamlit.
I found a BMI calculator on the web and replicated it in Streamlit. You can see my version of the app here.
I’ve put the code on GitHub with full instructions so you can start playing with Streamlit.
HTH,
Arvindra