I am not the expert here... but don't you need another else statement to cover all other conditions?

we used softwareupdate to check and install patches on all of our devices.

Why not use smart groups to scope the macOS versions you want to target and apply that to the policy?

Add "fi" after "elif" and "#" before anything on first line. Should work then.

#!/bin/bash
sw_vers

if [[ "$ProductVersion" == "10.13.6" ]]; then
softwareupdate -i 'Safari13.1.1HighSierraAuto-13.1.1'
elif [[ "$ProductVersion" == "10.14.6" ]]; then softwareupdate -i 'Safari13.1.1MojaveAuto-13.1.1'
fi
sleep 1

exit 0

Also can't find any variable defined, so I guess this script below is something you're looking for -

#!/bin/bash
ProductVersion=$(sw_vers | grep "ProductVersion" | awk '{ print $2; }')
​
if [[ "$ProductVersion" == "10.13.6" ]]; then
softwareupdate -i 'Safari13.1.1HighSierraAuto-13.1.1'
elif [[ "$ProductVersion" == "10.14.6" ]]; then softwareupdate -i 'Safari13.1.1MojaveAuto-13.1.1'
fi
sleep 1
​
exit 0

Also it would be good to add "else" to get some output as well if none of defined OS versions doesn't match in if statement, e.g. -

#!/bin/bash
ProductVersion=$(sw_vers | grep "ProductVersion" | awk '{ print $2; }')
​
if [[ "$ProductVersion" == "10.13.6" ]]; then
softwareupdate -i 'Safari13.1.1HighSierraAuto-13.1.1'
elif [[ "$ProductVersion" == "10.14.6" ]]; then softwareupdate -i 'Safari13.1.1MojaveAuto-13.1.1'
else
echo "OS version didn't match"
fi
sleep 1
​
exit 0