Hey there appleios100,
Thanks for reaching out to Apple Support Communities. I understand you want to use Personal Automation to schedule a message to someone. I’ll be happy to help with this.
For info on if you can do this, and how to create your personal automations, review the Shortcuts User Guide. This link has the first page of using personal automation and you can view the Table of Content at the top of the article for more sections:
Intro to personal automation
If you can’t use Shortcuts for this, you may consider setting yourself a reminder for this. Take a look at this article for info on how to use Reminders:
Use Reminders on your iPhone, iPad, or iPod touch
Cheers.