Hello, when I try to update the system I get this error:
The database is invalid or corrupted (pgp signature) Database synchronization failed
I followed this guide
but with the command
sudo pacman-key-Refresh-Keys
The command goes into a loop and the terminal has not started for 2 hours. What should I do?
This is not a valid command. 
And you have removed the community repo as described?
This is the command I entered
sudo pacman-key --refresh-keys
Sorry, I use Manjaro but I’m not an expert user. How can I fix this situation? Thanks,
Can I close the Terminal even if the command in question hasn’t finished yet?
That command is often very slow/takes a long time.
You could use another keyserver - but that would add another layer of complication for you as “not an expert”. 
Yes, you can cancel it - but then the keys will not be refreshed - which is what you wanted.
The command in question wasn’t that it was slow, it just stopped. What I need is to restore the system to work, the fact that I am a non-expert was the main reason that made me come to ask for the help of experts. Dismissing me with a “this is what you wanted” seems to me outside the Linux philosophy.
It was not a dismissal and certainly not meant as one.
To prevent further misunderstandings of similar type I’m withdrawing from this thread.
Good luck!