Looks like a typo which is spilling a warning.
Somebody has /sbin/modprobe -I whereas the correct form is /sbin/modprobe -i
In program options, that actually matters.
When compiling, you may get warnings and errors
Warnings is the compiler wagging a finger.
Error are show stoppers.
Ignore it. It carried on, so it's a warning.
In slackware, all packages are lower case, and in /var/log/packages. So this commend
will show you if the kernel headers are installed. Your compile dies very quick if they are not, as a rule.