I appreciate your help. Actually I was getting the error due to some JS code ingestion browser extension that I was working on. Switching to another browser helps me.
https://trailhead.salesforce.com/trailblazer-community/feed/0D54V00007ZB9KYSA1
2番のType add and select the Add Prompt Instructions element to add to the flow.が機能しない
Please make sure you have enabled this flow
Prompt Templateの削除
To delete flex templates that use flows or Apex classes, first remove any flows and Apex resources in all versions of the template. Second, delete any flows and Apex classes that reference the prompt template. Then delete the flex template.
https://help.salesforce.com/s/articleView?id=ai.prompt_builder_limitations.htm&type=5
新しいアクションを設定できない
。プロンプトテンプレートを有効にしていなかったため、アクションが利用できませんでした。