Yeah, that error message is not too helpful. There’s a syntax error in your query with A as distinctBrands.
I’m not picking up exactly what you’re laying down, but maybe a query like this might get you going in the right direction:
{
ITEMS as var(func: gt(Stem.weight, 39))
q(func: uid(ITEMS)) @groupby(Stem.brand) {
count(uid)
}
}