Extension Download Errors
two reasons I can think of:
- civicrm website was overloaded or down for a short maintenance operation
- your server or network connection prevents your site to connect to civicrm

if it's the former, wait a bit an retry, everything should be back to normal

to test if it's the later, try to connect to the site from the command line (if you have ssh access to your server) and run curl https://civicrm.org. if you have an error message, it should point you to the right direction

(we did change something about the TLS/SSL protocol accepted but don't recall if it was the main site or not). In any case, it would probably mean you have very old software on your server and you should upgrade your OS