What is it called when you can speak and read in a language, but you can't write in it?

It is called Chinese or 中文。

Oh you mean 提笔忘字 forgot how to write whenever you lift up the 筆. TG for Pìnyīn.

ps: some say the term is “illiterate” but in China a person who is illiterate is 文盲wenmang. 盲 is blind and it means that a person who is illiterate cannot read. Of course they can speak unless they are also dumb.

But if you can think you can still be literate but you need to have your eyes open to recognise the Word.