How old is your shortcuts.xml ? The Send Via Outlook was commented out in v7.6.3 in 2019, and those comments were removed completely in v7.9.3 .
That said, you are allowed to have whatever commands you want in your run menu, so you are allowed to keep the ancient outlook /a "$(FULL_CURRENT_PATH)" command if you want it. However, as with any program run from the command line, if it’s not in your PATH, you need to provide a full path to the outlook.exe executable. I have a working Outlook installation on my work laptop, but if I drop to cmd.exe or powershell and try to run Outlook from the command line by just typing outlook.exe, I get the error 'outlook.exe' is not recognized as an internal or external command, operable program or batch file. . This is the equivalent command-line message to the ShellExecute ERROR that you quoted.
For my new work laptop, with a clean installation of Outlook, outlook.exe is not in the PATH, so neither the command line nor Notepad++'s Run command can find it. But if I set Notepad++ > Run > Run… to "C:\Program Files (x86)\Microsoft Office\root\Office16\OUTLOOK.EXE" /a "$(FULL_CURRENT_FILE)", it will attach the saved copy of the active file to an email using outlook – please note the quote marks.
So you need to find your path to the outlook executable. (Probably something similar, though I’m guessing you have a sensible 64bit outlook at "C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE", but that is just a guess)