Hi RifatManazir,
I lost all my Keychain passwords on iPhone (it was locally stored in Accounts & Passwords - in Setting, I could use my passwords even the Keychain was off, but not the services - Home app, all apps that need the Keychain access were denied). However, after a few hours, all saved passwords were just vanished by itself.
So I decided to sign in my iCloud on someone’s MacBook, I skipped the importing stuff part and I only just turned Keychain on in there. I went to check on my iPhone 11 Pro Max, the iCloud Keychain was able to toggle.
Now my iCloud Keychain is on!
If you have an access to Mac OS, please use iCloud on the machine, the Keychain is just needed to be toggled on/off in there.
I hope this will fix your iPhone 11 Pro Max Keychain issue.
Regards,
Boz.