At step 4 I get the message: The config file could not be written.
I had changed the permissions of "includes/global.inc.php" to 0644 and it's green all right, but I still get the message: The config file could not be written.
What have I done wrong?

If it can't write to it, I would suggest that it needs to be 0777 until it is written.

The other option, would be to create a config file at the address below, and upload it to the necessary folder:

https://www.cubecart.com/build/index.php?step=1

Thank you for the swift reply.
It was me who got a path wrong
Now it's working.
Thank you any way.