The "No auth function available for given request" error should indicate that the excepted parameters for this call were not supplied correctly in the HTTPS request to /oauth2/token.
It looks like you're providing the necessary variables, but they may not be getting sent in the right way. How is the array in that second parameter to Http::post getting formatted and sent? The parameters for /oauth2/token should be sent as application/x-www-form-urlencoded POST parameters.
Are you able to print out the raw HTTP request? That may be useful for debugging this. We can't provide support for third party clients, platforms, or libraries though, such as spatie/flysystem-dropbox or Laravel, so I can't offer guidance on exactly how you'd do so. You may need to refer to the relevant documentation for that.