[HELP] DemoRPGMovement Jump

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Hello there! I am new to Unity and Photon and I am still learning. I need to include a "Jump" feature to the existing scene of DemoRPGMovement that is part of PUN. I have been trying to get it to work for days, and I did, but I have one problem. Everything works fine, but when I hit the "Spacebar" key, sometimes the character jumps, and sometimes it doesn't.

You can find my CharacterController code here: https://pastebin.pl/view/9983091d

I don't know what I am missing. Any help is highly appreciated!