Note: The Checks API only looks for pushes in the repository where the check suite or check run were created. Pushes to a branch in a forked repository are not detected and return an empty pull_requests array and a null value for head_branch.

By default, check suites are automatically created when you create a check run. You only need to use this endpoint for manually creating check suites when you've disabled automatic creation using "Update repository preferences for check suites". Your GitHub App must have the checks:write permission to create check suites.
Create a check suite