You're using the new experience version for document libraries in SharePoint Online.
You have a column for which the Require that this column contains information setting is Yes.
In this scenario, when you upload a file to the library, you're not prompted to enter a value for the required column. In addition, the file isn't checked out by default when you view it in the classic document library experience.
To resolve this issue, use the classic view for the affected library. For more information, see the following Microsoft website: