The default value for the api url is //api.github.com/repos/modxcms/revolution/tags.
Your server seemingly has a problem accessing this url.
Can you access the URL directly in your browser? Maybe by adding your github credentials? Maybe by using a VPN?
If you can access the URL, you can save the response (e.g. as “tags.json”), then put this file on a server of yours (e.g. https://mydomain.com/tags.json) and use this new URL for the setting ugm_versionlist_api_url.
(Of course this is only a temporary solution and you have to manually update the file every time MODX releases a new version.)