Error "Unable to translate Unicode character" when saving to txt file

Additional Information: It is not possible to translate the Unicode character \ uDFFF to index 195 on the specified code page.

I created an algorithm that results in binary values ​​(different lengths). I converted it to uintand then to characters and saved in stringbuilder, as you can see below:

uint n = Convert.ToUInt16(tmp_chars, 2);
_koded_text.Append(Convert.ToChar(n));

My problem is that when I try to save these values ​​in .txt, I get the previously mentioned error.

StreamWriter file = new StreamWriter(filename);
            file.WriteLine(_koded_text);
            file.Close();

What I am saving is: "忿 췾 ᷿] 볯 褟 بخي 痢 سى 伞 ِّ 㿯 ّ 翼 㐻 ك ك ك 筹 ﷿ 펿       which are some strange signs.

- txt. - , UTF8 , , . ?