Этот код AppleScript работает для меня, используя последнюю версию macOS Mojave.
property fileName : "Test Document.txt" --value can be changed
property theText : "Random Information" --value can be changed
writeToAFile()
on writeToAFile()
set theFile to (path to desktop as text) & fileName
set theFile to POSIX path of theFile
try
set writeToFile to open for access theFile with write permission
write theText & linefeed to writeToFile as text starting at eof
close access theFile
on error errMsg number errNum
close access theFile
set writeToFile to open for access theFile with write permission
write theText & linefeed to writeToFile as text starting at eof
close access theFile
end try
end writeToAFile