Install Webform on Drupal 8 : checkboxes greyed out
The new webform 8.5 versions requires the "Contribute" module. If you click the little expander next to webform you'll see its a dependency. Have a look on the webform project page for more info 
https://www.drupal.org/project/webform

Also just as a tip as I noticed from your question you've got your modules as /custom/contrib/webform its better if you do your module file structure as:

/modules/custom/

/modules/contrib/

with your own build modules in custom and anything from drupal.org in contrib.