I understand that other Game Pass titles are working, but that does not necessarily mean FC 26 itself is broken. These messages typically indicate that the EA app is not detecting the required game entitlement associated with the linked Xbox subscription.
Unfortunately, we cannot access your account, verify subscription entitlements, or manually add the game through the forums. This would need to be investigated by EA Help, who can check the Xbox account linked to your EA Account and whether the entitlement is being recognized correctly.
When contacting them, please provide the exact error messages, confirmation that your Game Pass subscription is active, and any case numbers from your previous contacts. I understand that you have already spent considerable time on this, but this is not something that we can resolve directly from a forum report.