From the FAQ:

Is there a way to “Skip” a log?

No, and this is very intentional, based on lots of research. Why? In short, because it hurts way more than it helps. We’ve tested it out, and hundreds of studies have confirmed our findings: Relying on willpower in the moment is a terrible strategy for building lasting habits. Once our brain makes the connection that “skipping” something has no negative effect (e.g. You go on vacation and skip a work out, but it doesn’t reset your Streak or affect your Success Rate), it becomes far too easy to skew your stats and the entire system breaks down.

So what do you do if you are on vacation or sick? The best option is to attempt to do the habit, even if it’s not to the same level you usually do, and log Yes since the act of keeping up the habit is what helps to build it for the long-term. You could also add a Log Note so you can look back later to see you were on vacation or sick at that point.

To go with the Work Out habit example, if working out normally means stretching, strength training and cardio, but you’re on vacation and don’t want to make time to go to the hotel gym, simply do your stretches and maybe some quick bodyweight exercises in your room and go on a walk during the day. If it’s a work habit, treat it the same way as a paid vacation policy; if you’re getting paid to take time off, you deserve to count your habits too. :)

There’s nothing wrong with logging Yes even if it’s not your normal routine, because the important thing is that you’re not completely breaking the habit. Keeping your habits top of mind and being consistent is what gets the best results.