Error using Subpage builder

Kirby 3
,
11

I used a fresh installation of Kirby, and retested using minimal blueprint files. I’m still getting those errors.

mainpage.yml

Title: mainpage
sections:
  introduction:
    headline: Area for adding new Particular pages
    type: info
    text: <b>Writing Particular pages</b> (below) is where pages are <b>added, edited,</b> etc. to the **Mainpage** page.
  pageslist:
    headline: Writing Particular pages
    type: pages
    status: all
    info: <br>Date published {{ page.datePublished }}
    layout: list
    parent:  site.find("mainpage/sub-mainpage")
    limit: 50
    flip: true
    templates: sub-mainpage

sub-mainpage.yml

Title: sub-mainpage

options:
  changeTitle: false

  subpage_builder:
    - title: Subpage
      uid: subpage
      template: subpage
      num: 1

columns:
  - width: 3/4
    sections:
      pieces:
        type: fields
        template: find("template/mainpage")
        fields:
          comment:
            label: Comment
            type: textarea
          tags:
            label: Tags
            type: tags
          createdBy:
            type: hidden
            default: "{{ kirby.user }}"
      additionalInfo:
        type: info
        text: Now go to the top left, and click on **Draft**, and publish the new piece.
  - width: 1/4
    sections:
      info:
        text: Use this <b>Subpage</b> section below to Add a Subpage Entry.<hr>When <b>Add</b>ing - the <b>new Title</b> should be the Subpage (site name).<hr>Remember to "Publish" the draft after you've saved the entry.
      subpage:
        headline: Subpage Creator
        help: Use this section to add a Subpage Entry,<br>Remember to "Publish" the draft when you've finished.
        type: pages
        limit: 10
        flip: true
        templates: subpage ## This uses the subpage.yml blueprint for entering data
        parent: page.find("subpage") ## This saves the entry under the subpage/ subfolder ?

subpage.yml

Title: subpage
fields:
  titleName:
    label: Title of Subpage Piece
    type: text
    required: true
    placeholder: "{{ page.parent.parent.titleName }}"
    help: {{ page.parent.parent.titleName }} <- Copy this and paste above if correct. <br>This **Title** is a required field (cannot be blank) - 
  titleLink: 
    label: Link of Republished piece
    type: url
    help: **Link** is a required field (cannot be blank)
  comment:
    label: Comment
    type: textarea
    help: To ensure this entry is Published, Scroll to the top and change <b>Draft</b> to Public