The error is indicating that there is an issue with the Bing Maps key you provided in the request. Double check that you added your key correctly, many accidentally clip off the last character or two.
Also, your request looks odd, there should be no curly braces around any of the parameters (points, heights, key). If those are there in your request, it's possible that is causing an issue for the request being parsed and thus the key is not being properly parsed.
Additionally you are not providing any value for the heights parameter, so that should be removed. Here is what your query URL should look like, just append your key to the end (I switched this touse https as that is a best practice):
https://dev.virtualearth.net/REST/v1/Elevation/List?points=17.75,83.34,17.75,83.34,17.75,83.34&key=
If the above doesn't work and key is 100% correct, try creating a new key and see if that works. Small chance there was some issue when the current key was created that might be causing an issue.
Side note, all of your points in the query are the same, so you are going to get the same result 3 times. If you are processing a lot of points across multiple requests, avoiding duplicates can help reduce the total number of requests you make, thus less transactions/costs, while reducing the processing time.