When I join my java code in HTML it doesn't work anymore

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Javascript

CSS

HTML

I would like to put this : (im french)

window.onload=function(){
		const facileBtn=document.getElementById("boutonF");
		const moyenBtn=document.getElementById("boutonM");
		const difficileBtn=document.getElementById("boutonD");

		function changeEnFacile(){
			console.log("ca marche")
			document.getElementById("calcul").innerHTML="c'est facile";
		}

		function changeEnMoyen(){
			console.log("ca marche")
			document.getElementById("calcul").innerHTML="c'est moyen";
		}

		function changeEnDifficile(){
			console.log("ca marche")
			document.getElementById("calcul").innerHTML="c'est difficile";
		}



		facileBtn.addEventListener('click',changeEnFacile);
		moyenBtn.addEventListener('click',changeEnMoyen);
		difficileBtn.addEventListener('click',changeEnDifficile);
	}

in a Javascript file and then put

<script src="file.js">

What I have tried:

I have to precise that the program works in the HTML "script" tag. But it doesn't work... suggest ?