If everything (Nucleus, iPhone, MacBook Pro) are connected to the same local area network, the Roon app on your iPhone should work very much the same as the Roon app on your MacBook Pro.
On both iPhone and MacBook Pro, under Settings => General, they should show connected to your Nucleus. Settings => Audio should show the same available outputs. What do you see under those two settings?
Thankyou, not able to see Nucleus under setting > General or setting >. Audio, nevertheless after abandoning effort last night this morning I am able to use iPhone as a remote thankyou