1.) Check the version of your cf cli (i.e. `cf -v`) and compare that to the recommended version of your target platform (i.e. run `cf curl /v2/info`, see `min_cli_version` and `min_recommended_cli_version`). Make sure your cf cli version is between the versions in the two properties.
2.) Check with your platform operator and make sure that the platform is working correctly & there are ongoing outages or issues.
3.) Run `cf -v logs app-name`. This should give you verbose output and show what happens with the WebSocket request. Here's an example of a successful request. In your case, it's likely returning a 4xx/5xx status code instead of the 101.
WEBSOCKET REQUEST: [2018-07-23T08:57:49-04:00]
GET /apps/<guid>/stream HTTP/1.1
Host: wss://doppler.run.pivotal.io:443
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Version: 13
Sec-WebSocket-Key: [HIDDEN]
Origin: wss://doppler.run.pivotal.io:443
Authorization: [PRIVATE DATA HIDDEN]
WEBSOCKET RESPONSE: [2018-07-23T08:57:49-04:00]
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-Websocket-Accept: [PRIVATE DATA HIDDEN]