"Error in call to API function "files/upload": This API function operates on a single Dropbox account, but the OAuth 2 access token you provided is for an entire Dropbox Business team. Since your API app key has team member file access permissions, you can operate on a team member's Dropbox by providing the "Dropbox-API-Select-User" HTTP header or "select_user" URL parameter to specify the exact user <https://www.dropbox.com/developers/documentation/http/teams>. "
You're getting this error because you're using an access token for an entire Business team to call a user-specific endpoint. You can do so, but you'll need to specify which team member to operate on. You can find information on how to do so under the "Member file access" section here:
Alternatively, if you don't want to connect to entire Business teams, and instead just want to connect to individual accounts (Business or not), you should register a "Dropbox API" app, instead of a "Dropbox Business API" app: