(or "In dash, ... is not supported." when using
#!/bin/sh i=1 echo "$((i++))"
Write out the increment/decrement operation explicitly:
#!/bin/sh i=1 echo "$i" i=$((i+1))
Prefix and postfix increment and decrement are extensions in bash and ksh. They're not supported in dash or POSIX sh, which is what your shebang says you're using.
If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning.
You can use
# shellcheck disable=SC3000-SC4000 to ignore all such compatibility warnings.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.