HI Tina
You can go to Customizing and make changes to Screen Layout but only at Document Type level
Follow the path below:
IMG>MaterialsManagement>Purchasing>Purchase Order>Screen Layout at Document level
To make Screen Layout with respect to Purchase Organization and purchase groups, you can pursue the creation of transaction variants via tcode SHD0.
Refer link below
Transaction Variant - A Step by Step Guide for Creation - Code Gallery - SCN Wiki
You can make more than 1 transaction variants and then can add them to different roles for your specific requirement. The thread below explains the same.
Can I have diff transacton Variant for the same... | SCN
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