This is my first time using Wolfram Mathematica and writing in TeX. I'm on the free trial version of Mathematica, and I'm writing in a notebook, using the "Inline TeX Input" option on the "Insert" tab. Previously, I have only ever written mathematics in MathJax.
I tried the above first without the dollar signs.
It didn't work then either. I figured maybe the problem was the lack of dollar signs. It appears not. A further confusing detail here is that the slash in front of the sum is red in the last picture, whereas it isn't in the one above. So I thought, maybe I have to add a dollar sign in front of all of them (not that that makes any sense to me, but I have no idea what I'm doing).
It still didn't work, but in this instance, the slashes aren't red anymore. I don't understand why this isn't working, according to this guide, sigma functions are displayed by writing "\sum".
EDIT:
I tried doing as instructed in the answer by @Nasser, but upon pressing enter, the box turned red. Here's the code:
Failure["TeXAssistantError", Association[
"MessageTemplate" -> "Errors: `Errors`",
"MessageParameters" -> Association["Errors" -> {}],
"Input" -> "crtl$ \\sum_{j=0}^{3} j"]]