Saving Group & Knockout or Knockout only builds a bracket. Only the tournament Owner and an Administrator can change it.
Everyone else is sent to the tournament's public page, which has no Format tab.
Reading the bracket
Open the tournament and select Format. The Knockout Phase card holds the bracket and its rounds.
Look at the first round. Both slots in every match read Add Team, because nothing has been decided yet.
Look at any later round. Its slots are already written for you, and you cannot edit them.
A bracket of eight teams gives you quarter-finals, semi-finals and a final. Four teams gives semi-finals and a final. Two gives a final.
Renaming and resizing the bracket
Select Edit on the bracket header.
Change the Bracket name if you want to. The name is used in every match title.
Open Number of teams to change how big the bracket is.
Select Save. The rounds are rebuilt to fit.
Deleting the bracket
Select the delete control on the bracket header.
Read the confirmation. Deleting a bracket deletes its matches too.
Select Delete.
If it does not work
The first-round slots are still empty. That is normal. Fill them yourself - see "Setting and renaming bracket matches in a football tournament".
A "Winner Match" slot will not open. It cannot be edited. It fills itself when that match finishes.
Resizing lost your slot choices. Changing the number of teams rebuilds the rounds. Set the slots again afterwards.
Your bracket is pre-filled with group positions. That is padel - see "Brackets in a padel tournament".