Can Teemo Strategist ability be used from hand if there are no units on the battlefield? If you want to find Nocturne...

No, you cannot activate the ability of Teemo, Strategist from Hidden if there are no valid enemy units at the battlefield.

According to the official ruling on this interaction:

• Ability Activation: When activating any ability in Riftbound, all targets must be declared as the ability is placed on the chain. If there are no valid enemy units to target at the battlefield, the ability cannot enter the chain at all. Consequently, the reveal and recycle effects do not occur.

Regarding Nocturne:

• Finding Nocturne: Yes, you can play Nocturne off Teemo, Strategist's effect, but only if the ability successfully enters the chain (i.e., there is at least one valid enemy unit to target). If the ability is successfully placed on the chain, you can reveal your cards and, if Nocturne is among them, use his ability to play him.