The attached screenshot is the installer page for selecting non-linux (windows) partitions. All choices are preselected, so this can be the problem only if you unselected them by your choice. In this case anyway you shouldn't have got the Windows boot option and thus fall into the boot.mngr error, so I don't think this is the problem. If you would like to proceed with further testing, you may install openmamba again, and then we can try to detect and fix the problem from there.
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