If one or more effects say you 'can' do something and one or more effects say you 'can't' do that same thing, the 'can't' effects always take precedence. For example, Queen Guinevere explicitly allows you to untap an allied minion, while Rhitta Gawr of Snowdonia prohibits other minions from untapping. If Queen Guinevere tries to untap any minion other than Rhitta, the effect will fail because the 'can't' effect from Rhitta takes precedence. Note that 'lose' effects (e.g. Sky Baron) also implicitly include a 'can't' effect. So if Sky Baron is in the realm and then you cast Power of Flight, the 'can't' effect takes precedence and the minion doesn't gain Airborne.