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 Discover it® Miles credit card. All your everyday purchases will earn 1.5 miles per dollar spent. What is more, Discover will match all the miles you’ve earned at the end of your first year (if you are a new cardmember). For example, if you earn 30,000 Miles, you get 60,000 Miles. Earned miles can be redeemed with any airline, including United Airlines.