1) The release buttons have not been pushed.
Release the tap unit by pressing the release buttons on both sides of the tap unit.
2) The tap unit is difficult to remove.
Beer residues may cause the tap unit to stick to the keg. Clean the tap unit with hot water and, if necessary, some washing-up liquid to dissolve beer residues. Then remove the tap unit from the keg.
3) The keg is still in the appliance.
Lift the two blue locking levers completely. Remove the keg and tap unit from the appliance and release the tap unit by pressing the release buttons on both sides of the tap unit.