ERRORin Cannot read property 'loadChildren' of undefined [ERROR] An error occurred while running subprocess

hi, even though my app is served by ionic serve, ionic build --prod fails with the error in the post’s title, this is what I get with ionic info:

<pre>
<b>Ionic:</b>

   Ionic CLI                     : 6.12.0 (/usr/local/lib/node_modules/@ionic/cli)
   Ionic Framework               : @ionic/angular 4.11.10
   @angular-devkit/build-angular : 0.803.28
   @angular-devkit/schematics    : 8.1.3
   @angular/cli                  : 8.3.28
   @ionic/angular-toolkit        : 2.2.0

<b>Capacitor:</b>

   Capacitor CLI   : 1.4.0
   @capacitor/core : 1.4.0

<b>Cordova:</b>

   Cordova CLI       : not installed
   Cordova Platforms : not available
   Cordova Plugins   : not available

<b>Utility:</b>

   cordova-res : not installed
   native-run  : not installed

<b>System:</b>

   NodeJS : v10.19.0 (/usr/bin/node)
   npm    : 6.14.8
   OS     : Linux 5.4
</pre>

any helps is appreciated