Why is the entry check switched off for my Telegram channel?

The entry check is available for groups only, so in a channel the switch stays off and greyed out. The reason is simple: in a channel subscribers do not post, only admins do, so there is nothing for a check to protect. The card in Settings → Organizer tools → External communities tells you whether each community is a group or a channel, and tapping the disabled switch opens the same explanation. If you want the check, apply it to the discussion group attached to your channel rather than the channel itself. Everything else on the community card, including marketing broadcasts, works for channels as usual.