Only Platform Admins Can Build Matchgrids

The "Build" button for building a Matchgrid is located at /match/matchgrids, which is only accessible to Platform Admins. Add a button reachable by Matchgrid Admins, perhaps from the "Manage" page (/matchgrids/manage/x) and/or the Attributes index (/match/attributes?matchgrid_id=x).

Note "build" permissions is already available to Matchgrid Admins, so this is just exposing the UI element.