When encountering 403 Forbidden errors in context of ACC/BIM360, the first thing that you have to check is "the provisioning".

Since ACC/BIM360 environment contains sensitive customer data, your ACC/BIM360 admin has to whitelist (a.k.a. provision) your Client ID.

Please refer to this blog post for more details: https://aps.autodesk.com/blog/bim-360-docs-provisioning-forge-apps