Loginisė negalimumasįmanomybė

Just little different and literary form but your original is also understandable and it can be used

Šia tema yra negalimumo loginio gerageras loginės neįmanomybės pavyzdys: Jei galiu aprašyibūdinti šiuos jausmus, tai nėra "nejaprašysiibūdinamai".

"Šia tema" is more a form of "with whom", like you are using this topic as a tool but in this case I think it's better to use "what" form like describing an object - "this topic"/"ši tema". In the original there is a word "prompt" but I think that your way of translation is also okay. Word "describe" in this case is theoretical, you can describe something by saying it, you can also describe by writing it on paper. So in your case it was like you only describe it by words while you writing. "Apibūdinti" is more open way, because you can "apibūdinti" something by saying it and also by writing.
"Neįmanomybė" can be a tricky word but it's a very literary version of "impossibility", your previous way was also okay and usable.
And there is no such a word as "nejaprašysimai". Closest form of the word I hope you wanted to use is "neaprašomas" - "not able to write by words". But as I suggest you to use "neapibūdinamas" as not describeable so just leave it.

Jei jie tikrai yra nejaprašysimai, taiibūdinami, tuomet negalėsiu parašyti teksto.

That word "nejasprašysimai" do not exist, so it's "neapibūdinamas" but plural form, because we are talking about group of words and they are indescribeable so it's just an adjective form.