It means that processing of the JS is defers processing of the script until after the rest of the page is processed. Why it’s causing the error is because other scripts the require the presence of the ACF scripts are running before the ACF script is processed. ACF scripts cannot be deferred, and this also applies to any scripts that ACF depends on, like jQuery.
It really depends on what other scripts are being loaded. You could attempt to be more specific about what scripts are being deferred, but I can’t say if that will help. The problem really is that because they are deferred the scripts may not run in the correct order.
