Can't set a display page template for a global structure

Issue

• We wanted to set a display page template in our global structures' default values, but we don't have the option as we do on other sites

Environment

• Liferay DXP 7.2 Fix Pack 17+
• Liferay DXP 7.3 Update 4+

Resolution

• The behavior is intended, you can't set a display page template for a structure or web content on the Global site because there are no pages you would be able to set.
• There is a Feature Request ticket you can follow: LPS-151080
• You could still do something like that (setting a display page template from other sites) in the earlier patch levels of Liferay, however, LPS-139624 has removed this option to fix that unintended behavior, since you shouldn't be able to do that from other sites either.