The doc provides the supported syntax:
https://dgraph.io/docs/query-language/sorting/
q(func: ..., orderasc: predicate)
q(func: ..., orderdesc: val(varName))
predicate (orderdesc: predicate) { ... }
predicate @filter(...) (orderasc: N) { ... }
q(func: ..., orderasc: predicate1, orderdesc: predicate2)
ordering by val(var) and by a predicate is not supported.
Could you create an enhancement request?