Hi @William Wallace,
Per my test, I could create a planner in sharepoint and add it in page
You could refer to the following steps in the document to create a new planner
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.