Cannot Tags and Groups fields in Webform CiviCRM Integration - Drupal 8
I suspect d8 webform_civicrm does not yet have those fully functioning. KarinG is leading the project and would welcome assistance and/or funding I suspect. I think the most recent update is in this blog where she says


  Drupal 8 port! The D8WFC project kicked off in August 2018 and we've already got Contacts, Memberships and ... Contributions working. Yes! We're transacting through live pathways. Try out the dev version: https://www.drupal.org/project/webform_civicrm/releases/8.x-5.x-dev if you feel adventurous: it currently contains Contacts and Membership functionality. If you want to try Contributions you'll need to check our GitHub repo:  https://github.com/colemanw/webform_civicrm/tree/8.x-5.x - and keep an eye on PRs against that branch