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These are not the package names, which explains why apt can't find them:

E: Unable to locate package ros-melodic-perception-pcl-ros E: Unable to locate package ros-melodic-perception-pcl-conversions

This _should_ work without any additional fiddling: apt-get install ros-melodic-desktop-full

So it's likely your system install is borked in some way. Try using aptitude instead of apt-get, which is much more clever about telling you what's wrong and how to fix it.