I understand this thread is SUPER old. But leaving an answer here anyway fpr anyone else looking this up.

Once you initially install nethunter using the command ./install-nethunter-termux it might just go straight back to the termux command line with nothing else. Then when typing in nethunter or nh (the shortened version that works as the same command), it says command not found.

At the point, run the ./install-nethunter-termux command AGAIN, but this time when you install it will say that it already has a file found, do you want to delete and re-download it. Click N. This will force it to basically unpack and use the root file previously downloaded.

Give it a few minutes to do its thing and you should see the title screen complete.

Now try running nethunter in the command line and Bing bang boom.

Hope this helped