Windows is unable to write to the plug-in folder.

Windows 10/11: – “I’m using SynthEdit on 10/11 and it works fine right up until the point where I save my synth as a VST”.
In Windows you can escape this file system limitation by running Synthedit as the Administrator. Open up the start menu, navigate to SynthEdit 1.4/1.5, then right click on the icon and select “Run as Administrator” from the menu.

Note: It’s best to set SynthEdit to “Run as Administrator” rather than using the method below.

If you get a message saying unable to write VST plug-in folder the reason is that
by default, Windows does not allow saving files to your VST Plug-ins folder. This prevents SynthEdit exporting VST plug-ins.
You can fix this by changing the folder’s Permissions:
1) Use Windows Explorer to browse to your VST Plug-ins folders. These are usually:
C:\Program Files\Common Files\VST3
C:\Program Files\VSTPlugins
2) If the folder does not exist, create it.
3) Right-click the folder – select ‘Properties’
4) Choose the ‘Security’ tab.
5) Click ‘EDIT’ (You may need to click through a UAC security prompt).
6) Select username “Users”.
7) Tick the options to Allow ‘Write’ and ‘Modify’.
8) To finish Select ‘OK’ to close the two dialogue boxes.