A cloud-based identity and access management service for securing user authentication and resource access
I think test‑b2c has its GA‑root gone, so just try switch‑dir→b2c→Users→add‑guest once; If it won’t bind, open a support ticket for that b2c, choose tenant lockout, and upload domain‑ownership proof so MS can restore a GA.
If even that fails, request a full tenant‑lockout recovery under that b2c.