Add the following at the top of the script file android-pin-bruteforce.
#!/data/data/com.termux/files/usr/bin/bash

Did this solution help?

Yes. I had it working in Termux on a rooted Moto G7 Play in 2021. Don't recall if I needed to do the modification listed above, but if you run into errors, try it.

Recommend closing this issue and possibly adding a section to the readme that lists devices/builds (i.e. Nethunter, Termux, etc) it's been verified to run on.