Which of the Garmin mobile apps is this? I don't even know what "enable check-ins" MEANS.
Check-in messages are the crippled-up version of preset messages. There are 3 of them. Each has fixed text which you cannot modify, All three of them have the same recipient list, which you set at explore.garmin.com. (The web site makes it look like you can have separate recipient lists. But when you modify one of them, they all change.)
[Preset messages were the original "free" messages. You could set the text of each preset You could have a different recipient list for each. Garmin did away with all of that in favor of check-ins. Because who doesn't like LESS functionality? /rant]
If you have an account which pre-dates the "improvement" to check-in messages, you have the option to NOT convert to check-in messages. (At least, you used to have that option. No idea if that's still true.) The conversion is a one-way street. Once you convert to check-in, you cannot change back. I only mention this because that MIGHT be what the app is talking about. Or not.
If you have an older account, make your decision at explore.garmin.com. If you have a newer account, you are stuck with check-ins.
If you really want to know what this is about, open a support ticket.
