Would it also work with "楼" instead of "层"?
Yes, "樓" should also be accepted, but it's currently rejected.
Is it ungrammatical to put 有一共 instead of 一共有?
It cannot be that my correct answer is wrong just because I forgot to add period at the end of the sentence :(
It says "我的办公室一共有二十三层。" used the wrong word and that "我的办公室一共有二十三层。" is correct — in what way do they differ?