This issue occurs when the LMS connection on H5P has the wrong Client ID and/or Issuer.
When the wrong information was used to fill in the Connection Settings, the error message below will show when trying to insert content into the LMS.
To fix this, you need to change the Connection Settings (Client ID and Issuer) on H5P.com to the same values provided by the LMS.
It should look something like this:
Note: this is how the Client ID looks on an LMS.