Magento – Magento 2: how to send POST request to REST API

Not sure if this is the problem for you but I had a similar issue the other day.
My problem was actually 2 problems.

1. The authorization url for admin actions should be ROOT/admin/oauth_authorize not ROOT/oauth/authorize like it is for guest or customer requests.
2. An admin role should be created in the backend and at least an user must be assigned to it. For that go to System->Web Services->REST Roles click on Add Admin Role.Set the permissions and after the page reloads you should see a tab called Role Users. Click on Reset Search and add at least one admin to the role. Save and try again.

I hope that at least one of the points above solves your problem

So @user2636834 did you figure this one out already?

I remember when I was setting up a test scenario that I got the sames error(s). Seemed that I didn't have Oauth installed :O. So in short I did (running Ubuntu and following these instructions, also running into this error):

pecl install oauth

Then add extension=oauth.so to php.ini's "Dynamic Extensions" section. Then restart Apache

I tried the examples from the Magento site and got an error from the API:

"Invalid auth/bad request"

I had to add a 'Accept' => '*/*' header to the $oauthClient->fetch() call. After that it worked all just fine.

Hope you can get things running if you hadn't already.