How can I minify CSS and JS files on my website?

If you look at the top of your file it says:

/** Shopify CDN: Minification failed

Line 16:0 Transforming class syntax to the configured target environment ("es5") is not supported yet
Line 17:13 Transforming object literal extensions to the configured target environment ("es5") is not supported yet
Line 22:6 Transforming const to the configured target environment ("es5") is not supported yet
Line 30:0 Transforming class syntax to the configured target environment ("es5") is not supported yet
Line 31:13 Transforming object literal extensions to the configured target environment ("es5") is not supported yet
Line 35:4 Transforming const to the configured target environment ("es5") is not supported yet
Line 44:19 Transforming object literal extensions to the configured target environment ("es5") is not supported yet
Line 53:22 Transforming object literal extensions to the configured target environment ("es5") is not supported yet
Line 59:10 Transforming object literal extensions to the configured target environment ("es5") is not supported yet
Line 63:14 Transforming object literal extensions to the configured target environment ("es5") is not supported yet
... and 32 more hidden warnings

**/

Shopify can’t minify es6 as of today (Aug 2023)