"The request to create a scratch org failed with error code: SH-9999."

If you haven't already done so, there are a couple of one-time things you have to do in Setup on the source org.  Under Development:Scratch Orgs.  You have to turn on the " Enable Org Shape for Scratch Orgs" option, and you have to put the dev hub's org ID in the permissions text box below that.  (You have to do that even if the dev hub is the same as the source org.)