I tried to install a macOS High Sierra update, but I got the error "An error occurred installing macOS" and thus I decided to erase the Installer.app and dmg on my drive. Now every time I close my Mac it shuts down and tries to boot from the macOS Installer boot, but because I erased the update I get the "No Symbol sign". I restart my mac and use the power key + Option to boot into my actual boot, but I don't seem to find someone who knows how to erase that boot, is there a way you can help me out?
At the Startup Manager (accessed with holding alt/option at boot), with your macOS volume highlighted by the up arrow, hold down ctrl (which transforms the up arrow to a circular arrow) then choose the macOS volume. This will set this volume to be used as the startup disk in the future.