If a baron bit cannot be placed, does baron nasher still enter into the pit?

Based on the text of Baron Nashor, the conditional phrasing addresses your question directly.

Ruling: If the Baron Pit cannot be placed on the board, Baron Nashor does not enter it.

Reasoning

1. Card Text: The text for Baron Nashor states: "As you play me, add the Baron Pit battlefield token to the board if it's not there already. If you do, I enter there."
2. Conditional Mechanics: The phrase "If you do" creates a specific condition for the second part of the ability. Because the ability explicitly links Baron Nashor's entry into the Baron Pit to the successful addition of the Baron Pit token, the Baron cannot enter the Pit if the Pit token fails to be added to the board.
3. Chain Resolution: When you play Baron Nashor, this triggered ability is placed on the chain. During the finalization of this ability, the game evaluates whether the Baron Pit token can be added. If it is added, the second part of the instruction ("I enter there") is fulfilled. If the token is not added, the condition "If you do" is not met, and the Baron does not move to that location.

Note: I could not fully verify this ruling against the retrieved rules.