remove last quotes, -o and result file name shouldn't be in quotes or enclode
file name only, like this:
"C:\msys64\ucrt64in\g++.exe" "%File%" -o "hello"

Test your command line in Windows CMD (command interpreter)

-- 
<https://forum.pspad.com/read.php?2,78535,78536>
PSPad freeware editor https://www.pspad.com

Odpovedet emailem