Today, my manager told me that I must work over time to make up for a lack of planning and project management. Instead of incentivizing this unpaid mandatory overtime, my manager has made it clear I have no choice in the matter.

Is this a sign of an ignorant manager or just a disrespectful one?

What are possible causes of being placed in this position, and how might I avoid it in the future?

As a side question, what are good ways to get an employee to work overtime if you can't directly pay them? Good incentives, etc.

From what I hear, gaining your employee's respect is the single best way to get your employees to work over time, although you should never make a habit of it.