Where is the quantize button on Engine DJ 4.3.0

Engine DJ Desktop is library preparation software, not performance software. Well, not to say you can’t use it as hobbled performance software as @Parysatis has posted (https://community.enginedj.com/t/engine-dj-as-a-performance-software/58200).

There is quantize in the desktop version to assist in setting cues and loops. Choose the gear symbol on the top right and Cue/Loop Quantization in the Performance tab.

If you mean Engine OS on the players, then @djdragon has answered that.