A few things.

Off the bat the URL you are using to authenticate is depreciated, Try instead using /api/authentication/sign-in the version above only seems to work on older ALM instances

You will also need to make sure that you send your username and password as a a basic auth base64 encoded header

It would look like this:

Authorization: Basic cWNhZG1pbjpxY2FkbWlu

To get the base64 encoded username and password which is "cWNhZG1pbjpxY2FkbWlu" above I went to a base64 encoding site on the web(there are tons of them) and then you put in the username and password as username:password, so in my case I put in "qcadmin:qcadmin" since my username and password are the same and then base64 encoded it which gave me "cWNhZG1pbjpxY2FkbWlu" then you put the word "Basic" in front of it so that it is known that the authentication type is "Basic Auth"