You can only link a channel if your own account has two-factor verification (TOTP) turned on with an authenticator app (Google Authenticator, Authy…) — it’s an unavoidable requirement: the same code your app generates is what proves, over the channel itself, that it’s you.
If you don’t have it on yet, turn it on first in Settings → Security, at the bottom of the screen, in the “Two-factor verification” card — full procedure in Manual 26 · Settings and integrations, section 5 (scan the QR code or enter the secret, confirm with a code, save the 10 recovery codes). This manual doesn’t repeat those steps.
While you don’t have it on, the “Channel-linked access” panel on your Profile reminds you and keeps the link buttons disabled: