You re-initialize A to Π on every loop iteration, so it will never converge — you square it only once and then re-initialize it.
(I assume by \Pi you mean Π, i.e. you meant to tab-complete the unicode symbol.)
That is, your problem is not (just) Julia syntax, but rather you are struggling with the basic logic of the loop. There are a variety of programming tutorials that you might find helpful, e.g. the book Think Julia.
if A ≉ Π * Π
A = Π * Π
You can copy-paste this, and it’s good to know, that you always can, in (whole or in part), e.g. if you do not remember the correct TAB-completion.
That’s good advise, you can use this in function form or even better, type \napprox then press . The prefix \n for not (usually but not there !) does not work as \not (but allowing either way, a PR for, would be welcome, and LaTeX completions DO sometimes allow for two forms already). You can type \approx for ≈ but in that case, as for the functional form (and some ASCII operators), you can’t prefix with ! for !≈. That’s likely by design, to not allow more than one Unicode for (after TAB-completion), plus the ASCII form. Julia is in general case-sensitive, also e.g. for \Pi and \pi.