To start a knockout bracket, you need to have a power of two (2^n) number of teams, like 2, 4, 8, 16, 32 or 64. Otherwise you don't end up with two teams in the final. That's why brackets of other sizes are not available. There are two ways to fill up any remaining empty spots in your brackets, depending on whether you play a group phase first. Let's look into two examples.

Click on an empty spot in the bracket and toggle the switch arrows behind a position. Now, when starting the phase the system will automatically compare the teams with the same rank across all groups (based on points > goal difference > goals scored) and picks the best placed teams. This will work for selecting the best second-placed teams (best runners-up), best third-placed teams, and so on.

When you start a tournament with direct knockout but you're missing teams to fill up the entire bracket, you can give selected teams a bye in the first round. Those teams are automatically advanced to the next round, without having to play. Select bye as the opponent for high-ranked participants or perform a draw to assign byes randomly.