$0can't be assigned this way, and there is no portable alternative.
$0 can not be changed in a portable way.
You appear to be trying to assign a new value to
$0 in a
There is no portable way to do this. Write around it, or switch to Bash.
If you instead wanted to compare the value of
$0, use a comparison like
[ "$0" = "myname" ].
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.