How can I activate the "free of postage from" function in the group?

Go to System Settings → Administration → Settings → Basic Settings → System and check "Calculate postage".

Create a price group under → Administration → Settings → Groups for postage free from e.g. 50.00 EUR value of goods.

Create a shipping article/postage article (if not already available). Activate the article option "Article is postage" for the postage article.

Check in the master data (Master data → Select customer → Payment conditions/taxation) whether the check mark for "Fix payment conditions:" is not set. If it is checked, the price group settings will be ignored.

Assign the newly created price group "Free shipping from 50.00 EUR" to the customers. Just set the checkmark, you don't need to save here.

If you now create an order and add shipping costs as an item, they will be set to 0.00 EUR from a (net) value of goods of 50.00 EUR.