DSYEnvMgt failed: rc= 3221225781 can not finish installation

The installation process stops at this error point.

Background: I had the 2025 Solidworks on the machine, but then it failed to update to 2026. I tried to delete all SW on the machine (using an uninstaller) and start again, but now have run into this problem, and now I'm stuck.
What do I need to do to complete the installation?