Currently, the Mileage Plus card comes with the 30,000 bonus miles. To get this bonus offer, you will need to spend $1,000 on purchases in the first three months. As for the 50,000 bonus offer you wanted to get, we assume that it had expired by the time you applied for the card. If you need more details, please contact Chase, the issuer of the Mileage Plus cards.
As an alternative, you can consider getting a regular credit card that will allow you to earn miles on all your purchases. For example, you may like the Arrival Plus credit card from Barclaycard. The one-time bonus is 40,000 miles and you will need to spend $3,000 within first three months to get this bonus. All your everyday purchases will earn 2 miles per dollar spent and you will get 5% miles back toward your next redemption every time you redeem your miles. Earned miles can be redeemed with any airline, including United Airlines.