I think you can use SetNquads,
pb := fmt.Sprintf(`<%s> <%s> <%s> .`, uidRepos.UserIDs[0].UID, relation, uidRepos.UserIDs[1].UID)
_, err = txn.Mutate(ctx, &api.Mutation{
SetNquads: []byte(pb),
})
I think you can use SetNquads,
pb := fmt.Sprintf(`<%s> <%s> <%s> .`, uidRepos.UserIDs[0].UID, relation, uidRepos.UserIDs[1].UID)
_, err = txn.Mutate(ctx, &api.Mutation{
SetNquads: []byte(pb),
})