Have you spotted United’s “Mile Play” promotion in your inbox? If you’re eligible, booking “x” trips by January 1, 2018 you’ll get “y” miles.
You can see if you’re targeted for the promotion by checking United’s Mile Play offer page here. Then check out the airlines-mileageplus/1881108-new-promotion-mile-play-targeted-book-travel-x-trips-12-jan-2018-get-y-miles.html”>FlyerTalk thread on the topic to see what others have been offered.
While the “Unfortunately, the account number you entered is not eligible for this offer” message seems to be fairly popular, we did spot an offer for 150,000 miles for four booked trips and one booked trip for 68,000 miles. So it’s worth a try to check!