There are two training videos on Maslow’s Inferno YouTube channel, one for C++ and the other for Blueprints. Perhaps you will find a solution by viewing one of them.
(YouTube) How to Make Super Mario 64 Double & Triple Jump in Unreal Engine 5
*as Tuerer wrote above, you can simply increase JumpMaxCount to 2.
(YouTube) Unreal Engine 4 - Double Jump In 1 Minute (Tutorial)