It sounds like Endpoint is unable to write to Elasticsearch. In Fleet, is the Agent in a healthy state? Does the rest of Elastic Agent work correctly? For instance, in Fleet if you select the relevant Agent then the Logs tab (next to the Agent details tab) do you see logs being streamed to Elasticsearch?
Assuming Agent is working properly and it's just the Endpoint integration that is failing, a first step is to test Endpoint's connection to Elasticsearch. To do that, on the host where you're testing with EICAR, as an Administator, run the command "c:\Program Files\Elastic\Endpoint\elastic-endpoint.exe" test output. The output from that will hopefully give some indication of the problem.