The carousel is initialised in /js/scripts.js line 1276 in function Sliders()

$('.carousel').owlCarousel({
loop:true,
margin:20,
autoHeight:false,
navSpeed: 600,
nav:true,
responsive:{
0:{
items:1
},
479:{
items:2
},
1024:{
items:3
},
1466:{
items:3
}
}
});

and this is the complete list of options:

https://owlcarousel2.github.io/OwlCarousel2/docs/api-options.html

so you should use

autoplay: true,

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