Dojo Build 1.17.3 Closure compiler and string literals SyntaxError: illegal character

I commonly use string literals in a number of modules and no matter how simple the simple literals is I am getting this error

"Failed to evaluate module tagged as pure AMD (fell back to processing with regular expressions). module: xxxxxx ; error: SyntaxError: illegal character"

An examples of the string literals I am using that is throwing the error:
path=`path/${x}`;

I have in the my profile:

basePath: "./",
layerOptimize: "closure",
optimize: "closure",
releaseDir: "./release",
hasReport: true,
optimizeOptions: {
languageIn: 'ECMASCRIPT_2021',
languageOut: 'ECMASCRIPT_2021'
},

I thoughts this would set the compile to ES6, but I am also having issue to defining variables using "let".

Is there other settings in the profile that I should look at?