When editing a Page using the Avia Layout Builder, Content Element called Blog Posts I would like to only display posts which contain a Tag or an Author, please.
You can display the posts which contain a tag. Select the “Display entries from a custom taxonomy” and select “Post Tag” in the second dropdown. Then select one or more tags
There’s no feature to display the posts from a certain author though. You can try a workaround and use tags or categories instead or link to an author archive page (looks like: http://kriesi.at/themes/enfold/author/kriesi/ )