Hello there, rasmey.keo.
If you're trying to create a template for the purchase order, the option is currently unavailable. The design you see when you print the purchase order is based on the standard invoice template, but it cannot be specifically customised.
To change the style and color, you can follow these steps:
Then, go back and print your purchase orders.
For creating and customising invoice templates, you can check out the instructions in this article: Email and customise sales form.
Please let me know in the comments below if you're referring to something else. I'll be around to provide details and steps. Take care and stay safe.