Your initially budget is 100, so your 11 players must cost less than that. If you run out of available money before filling all positions, you'll need to choose less valuable players in some positions. One of the ways to do this is to use the price filters that are available on the 'Team Manager' page

Remember, the Fantasy Football value of players goes up and down: someone you could afford a few weeks ago may be out of your budget now - and vice versa. Similarly, if you sell someone who has gone up in value since you bought them, you may then be able to assemble a squad that would have cost more than your initial budget of 100.