The following error appears during installation, even if the previous existing version was already uninstalled via the Windows control panel:
"Another version of the product is already installed, the installation of this version cannot continue."
First, perform a clean reinstallation of the product by removing left-over folders from the previous installation and cleaning registry, using the steps mentioned in below article:
How do I perform a clean uninstall and reinstall of the Hydraulic and Hydrology products?
Another possible cause could be issue with the Microsoft visual C++ 2015 or Access Database Engine, which was case with one user. Try uninstalling existing Microsoft visual C++ 2015 and the Access Database Engine, then try installing again. Also, try temporarily disabling your antivirus/security software in case it is blocking installation (this was reported in at least one case).
Downloading OpenFlows | Hydraulics and Hydrology Software
Error when installing program: "Unspecified error / 0xfffff95a" or "Fatal error during installation / 0x80070643"
Error 1152 during installation