Nextcloud Version: 24.0.4
OS: Ubuntu 20.04
hi @Dipankar_Datta welcome to the forum 
you are missing the required support template. Please fill this form out and edit into your post. This will give us the technical info and logs needed to help you in case the machine in question in spitting out errors! Since you’ve provided zero technical details it is impossible to help you as-is. Thanks.
beside this NC 24 is outdated and I highly recommend you to upgrade!
It can happen when the web-app container running NC (or the host if it’s not in a container) doesn’t have outbound Internet connectivity. The most common way I’ve triggered this is by forgetting my NAT or firewall config in a new environment (or messing up my Docker stack networking).
Functioning Inbound connectivity != Functioning Outbound connectivity