https://wccftech.com/windows-10-1903-can-be-installed-on-pcs-not-meeting-storage-requirements/

Back in April, Microsoft updated the list of Windows 10 system requirements for the first time since the operating system’s release in 2015. To run Windows 10 1903, the Windows maker raised the minimum storage capacity to 32GB. While Microsoft didn’t officially give any explanation, we assumed it could be due to the newer version’s Reserved Storage feature that sets aside 7GB of space for Windows updates and system cache.

However, in an updated advisory, the company says that the 32GB requirement doesn’t apply to existing machines as anyone can update to Windows 10 May 2019 Update. The requirement is only for OEMs manufacturing new PCs.