I’m glad you fixed it. Though is a mystery why the file was missing there.
I’m a noobie too. And i thank you for the oportunity of learning more about this too 
gpg: can’t open ‘endeavouros-2019.07.15-x86_64.iso.sig’: No such file or directory
It means the file is not in the same folder where you’re running the terminal. (or doesn’t exist at all)