The "Clone" screen in SourceTree does not let you retry the operation if the URL could not opened successfully.
This screen asumes that if there was any error getting the repo using that specific URL, then the URL must be wrong. That, in itself, is wrong - as network connection issues could have caused the problem, not just the URL.
One such scenario is having your repositor(ies) behind a VPN at work. You might have forgotten to connect to the VPN in your machine, and then try to connect to the repo to clone it - the process will obviously fail, and then you go and connect again to the VPN, but the screen won't let you try again - you have to navigate out or close SourceTree, then enter again and re-enter all data on screen again.