If I lost two days of challenges, does that mean that the case is lost and I should withdraw from the challenge? Because not doing two or three times a day to fill in the gaps
Hi @mark1998 Challenges are just some extra spice to your training. As long it is not an official Freeletics challenge, there won’t be any regard (like a special badge) by successfully finish them. In any case a missing day/rep/what ever is still better (regarding your training goals) as missing all of them 
In your case, I personally won’t overthink the missing days and still accept the challenge 
There is absolutely zero reason to care about a challenge since they don’t grant you any reward (expect official ones which sometimes grant a badge, but the last one was months ago). Instead, follow your training plan to achieve real progress.
Also keep in mind that you will get sick at some point, e.g. due to a cold. So you will naturally miss something. But that’s normal and not a problem. It is long-term continuity which makes you strong at the end.