https://editor.p5js.org/julian2/sketches/Hk2qE_HY7
You’re allowed to write code with async/await (your browser should definitively support it), and it works as it should (as promised). But I believe it’s just the editor here that displays the async/await syntax as an error (even though it’s valid syntax)! I think it comes down to p5js only supporting a certain version of ecmascript? (citation needed!), and since async / await is pretty new (somewhere between es2016 and es2017? I’m not sure exactly when! 
) then it hasn’t been added to p5js’ syntax highlighting / static code error checker.
Since we’re running code in the browser / clientside, it should really just be a matter of browser support & async/await has been supported by all major browsers for a while! 