Learn English – I couldn’t catch it, but what is wrong with this sentence

You can definitely add words in between as much and as. In your simplified examples where nothing goes between them, it's because the words are implied:

I'll eat as much (food) as (it is) possible (to eat).

I'll clean up as much (of the mess) as I can.

There are two problems with your sun screen examples: 1) the phrasing inside the brackets is awkward and doesn't sound right to a native speaker 2) you're using much with countable nouns (sun rays) when you must instead use many.

A proper way to say this using as much as could be:

I'm wearing sun screen to cover up as much of my skin as possible.

The preposition within has nothing to do with the ordinary sense of the preposition with = “accompanying, alongside, by means of, etc.”. Within means “inside” in spatial, temporal and figurative senses.

The hoard was discovered deep within the mound.
You must respond to this communication within thirty days.
They are within their rights in refusing to be interviewed.

Consequently, within is not a valid translation of German in in this context. However, English does support both German uses: you may use either with for mit or in for in.

With this option ...
In this option ...

• HISTORICAL NOTE, for those who wonder why the compound with + in has this meaning:

  Old English wið (cognate with German wider) originally meant “against, by, back from”, and the modern sense of with was expressed by mid (cognate with German mit). In the later part of the Old English period, however, northeast England was occupied and to some extent settled by speakers of Old Norse dialects, and over the next centuries (down into the Middle English period) many Old Norse words replaced the corresponding Old English words. For instance, they and are are of ON origin. Old Norse við, a cognate of with and wider, had approximately the sense of mid and mit, and under its influence, with shifted to the modern sense. However, the old sense lingered in established compounds such as withstand (stand against), withhold (hold from), withdraw (draw away from)—and within (against or by the inside).