Sound like a syntax-error, but i can not recognize any syntax-errors in my shown code.
If there would be a Syntax-Error → it would be marked in RED

Did you also first imported the wix-data-api ???
import wixData from 'wix-data';
$w.onReady(async function() {
let RESULTS = await checkData();
console.log("RESULTS: ", RESULTS);
});
function checkData(){
wixData.query("Diamond")
.find()
.then((res)=>{return res.items})
.catch((err)=>{console.log(err);});
}
-Your Collection-Name is → “Diamond” ??? You have already double-checked it?
-Your Collection gives also the related permissions to enter your DATABASE ?
Check all your setup.
Use the CONSOLE of your browser for DEBUGGING.