The “Link” buttons are disabled. You don’t have two-factor verification on yet (section 2) — turn it on first in Settings → Security.
The bot replies “Invalid or expired code”. Your app’s 6-digit code changes every 30 seconds — generate a new one right before sending it (don’t copy it ahead of time) and check that your phone’s clock is properly synced. If the problem persists, confirm you’re linking the correct channel (the WhatsApp code and the Telegram one are the same type, but each one counts as an independent attempt against THAT channel).
A 2FA recovery code doesn’t work to link a channel. Correct, that’s the expected behavior: recovery codes (the 10 you saved when turning on two-factor verification) only work for password login, never for linking or opening a channel session — that always needs a live code from your authenticator app.
I sent my code, but nothing happens. It’s possible the number/chat you’re writing from is already linked to another person on your team — for security, the system doesn’t tell you about this case in the chat itself (so as not to reveal information to whoever shouldn’t have it). Confirm with an administrator whether that number already belongs to a colleague, or use the correct number/account.
My session closed and I can no longer operate from the chat. More than 30 minutes have passed with no activity (section 6) — send your 6-digit code again over the same channel to reopen it; there’s no need to link again.
Can I link the same channel (my same WhatsApp) to two different accounts? No — each number/chat can only be linked to one employee account at a time.