Is it possible to create system user via /access_token endpoint with Test User access token?

I am developing an application based on On Behalf Of framework. https://developers.facebook.com/docs/marketing-api/business-manager/guides/on-behalf-of

To implement the framework we need to create System User in clients Business Manager.

Here are the steps I did:

1. Create Business Manager with Test Users credentials. (POST me/businesses)
2. Connect Client Business Manager with a Business Manager that owns the Test App.
3. Create access token/system user for my Test App. (POST <business_id>/access_token endpoint.)
4. The call fails with "(#200) Permissions error"

However, this flow works with non-Test Users!

I wonder is some kind of a limitation for the test users? Or something extra has to be done?

I would appreciate any tips.