The macOS version of mktemp does not recognize the --suffix option.
Using pure -d should work since, it seems [1], macOS 10.11 however.
So to make the script work on macOS, stop using the --suffix option.
The downside is of course that the temporary dir will have an anonymous
name, but I see no risk of confusion given how short-lived the usage of
the dir is, and given the context it is used.
[1] https://unix.stackexchange.com/questions/30091/fix-or-alternative-for-mktemp-in-os-x