Welcome to the Make community!
There is no trunc function, Make removed it but forgot to update the documentation.
You can use the floor function instead.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!