Is the build process working ? If it’s working, this is a known bug due the Eclipse indexer synchronization. If you delete the errors or relaunch Eclipse, it should not appear, the error message should not appear. (I agree really confusing for users, and we’re looking for how to avoid it)
If not, it means the compiler has not been installed or cannot be executed. Please, verify that the plugin fr.ac6.mcu.externaltools.arm-none.win32 is installed in your Eclipse directory.