***CONTEXT***
I'm playing on a vanilla server with a couple of friends and we decide to add a few mods.
When showing how to install the mods to my friends one has a problem installing Forge and is faced with this message every time he tries to install the client.
I really don't know what to do since this has never happened to me before.
Link to error message: https://imgur.com/a/XwaAB
Thanks for any replies and sorry if this has an obvious answer.