how to install application to system user?

HI, I'm creating an app(META APP) that runs ads using the Business/Marketing API and then I have a question.
First, I created a system user (SU) in my business (B). After that, I want to install the app(META APP) to system user(SU) However, when I run post /applications, I get the following error: Unsupported request - method type: post. (https://developers.facebook.com/docs/marketing-api/system-users/install-apps-and-generate-tokens?locale=en_US#TOS)
The business app id is test app id. Access token is the test app administrator's access token that create via meta tool access token(developers.facebook.com/tools/accesstoken) System user is created with admin privileges. And In the web ui meta app, i can see admin user and test users, But when i request get {app_id}/roles api, i can not see admin user. only selected test users.
What should I do?