Hi @victoria I encountered the same problem, and letting the custom claim value as a single string does not work as well.
I tried to use Firebase - support for the custom claims edit - #18 by choward232 post's db.auth().getAuth() but it does not work as well, and it will generate a ReferenceError: createPool is not defined error
Can you help me check with the eng team that if admin.auth().setCustomUserClaims can be added as a firebase GUI mode function ASAP? Thanks!