It’s hard to tell just from the information you gave, but it looks like you’re running postcss-custom-properties to transform your CSS and it’s throwing an error. If you’re running postcss-custom-properties by yourself (i.e. it’s not called by some other process) I would try looking at that package’s GitHub issues to see if someone else has had the same error.
If your remote API calls are not working, you need to make sure your server implements CORS.
If local XHR calls are not working, try using cordova-plugin-wkwebview-file-xhr instead of cordova-plugin-wkwebview-engine.
If you get the UIWebView deprecated notice from Apple, search for the string “UIWebView” in your plugins. If a plugin is still using UIWebView then you need to use the latest version of the plugin if the maintainer has removed UIWebView code, or disable the plugin. (Note occurences of the string UIWebView in cordova-plugin-wkwebview-engine are fine - you can and should keep that plugin enabled.)
have you changed to IOS 5.1.1 at Monaca Building Settings?
It took me a while to figure that out.
I am running into a new problem now: with WKWebView, my app goes to a blank screen, and does not work.
Let me know if that works for you.