As the error message states Varnish cannot communicate with the content (Magento2) server.
If you are using health checks in your Varnish VCL, varnish will also show this error if the content server is "unhealthy".
To check your content server status login to the varnish server and enter varnish admin with the
Backend name Admin Probe Health Last change
magento2_php_apache_1 probe 10/10 healthy Mon, 22 Feb 2021
If you are not using the health probe check network communications between varnish and the magento 2 server (if they are seperate servers.)