Hi Archie,
The sandbox for that module expires after 1 hour, so if you wait until it expires you can Activate a fresh sandbox and complete the exercises again.
Please note you may have some behavior/errors you are not expecting if you use portal to access the sandbox subscription after expiration/re-activation. Reason is, the previous subscription will have been deleted and a new one created. Normally you can fix this by signing out/back in to portal and/or clicking gear icon in upper right and switching subscriptions.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP