This is a default behavior in SharePoint Online modern experience.
There is no OOTB way to show the properties pop-up while uploading documents as of now.
However, the list items/library files with incomplete/missing information will be shown in separate view called Items/Files that need attention like below:
Also it will show small message of Required info in column in list view (see in above image).
You can read more about this in my answer at: Page Library - new column with required field, how to prevent page creation if required field not filled
Additional: Microsoft is adding similar experience for SharePoint Pages library, you can check it here. It says:
A pre-publish panel enforces that authors supply missing required properties before they can publish a page or a news post. Additional improvements related to required properties in the pages library take effect.
I am not sure if Microsoft is planning to add this functionality for normal lists/libraries as well. If you want Microsoft to implement this then you can create a new idea at SharePoint UserVoice.