Shortcut to write to a file

See if it's not a permission issue for Shortcuts. Go to System Settings > Privacy & Security > Full Disk Access and add Shortcuts to the list of applications that can access your drive.

I tend to find Shortcuts and AppleScript somewhat frustrating, out past the sorts of tasks and GUI scripting those can do...

Here's an overly-chatty bash shell script that shows file creation, a test for writability, appending, and then overwriting:

#!/usr/bin/env bash
set -e
file="/Users/mrhoffman/Documents/testingfile.tmp"
if [ ! -e "$file" ] ; then
    touch "$file"
fi
printf "\n$file contains:\n"
cat "$file"
if [ ! -w "$file" ] ; then
    printf "Unable to write to: $file\n"
    exit 1
fi
printf "Hello, this is a test\n" >> "$file"
printf "Hello, this is another test\n" >> "$file"
printf "\n$file now contains:\n"
cat "$file"
printf "\n$file now contains:\n"
printf "Hello, this is the overwrite\n" > "$file"
cat "$file"
printf "\ndone\n"

To use that example script, launch Terminal.app and create a plain text file at the command line with that code, say with pico x.sh or such, and then (once) chmod u+x x.sh to permit the script to execute, then ./x.sh to execute it.