Question

Can I also create a challenge for a cache that does not exist yet?
Why I ask that:
I consider if I should create a cache "around the lake Biel". Since I have not fulfilled the conditions yet, this would be the perfect opportunity to learn how to create a checker.
Another advantage if this is possible: anyone can create their own challenges for their local rounds without having to create a cache.
Maybe with a hint in the description: There is no cache for this challenge yet. If you create one, you are welcome to use the checker.

Comments

Without more to go on than the above, I suspect your idea may not meet the challenge guidelines. You are not allowed to use arbitrary geographical limitations, only countries, regions and counties.

---

I agree with pinkunicorn -- this does not sound like it comports with the challenge cache guidelines.  

https://www.geocaching.com/help/index.php?pg=kb.chapter&id=127&pgid=206

Both of the following are examples of things that are not allowed, and they seem to apply here:

#10: "Challenges based on geographic areas other than countries, states/provinces, counties (or their local equivalent). For example, user-defined mapping polygons, latitude/longitude, radius, etc."

#12: "Finding all or any percentage of the caches, or of a cache type, rating, or other allowable value within a specified area; e.g. 80% of EarthCaches in France is not publishable."

If you want to hide several caches, put part of the coordinates in each, and then have a final "bonus" cache, that is acceptable for your own caches.

Answer 1

You actually have to. The challenge cache will not get published if it will not have a challenge checker. To request a challenge checker, you should write to the following forum:

https://project-gc.com/forum/list?5

Anyway, if you want to write the checker yourself, this is documented in FAQ: "How do I apply for access to write challenge checker scripts?":

https://project-gc.com/Home/FAQ#2616943827