Why am I getting an error from writing to the file I was able to read from?

This is a program to display 'frequent shopper'-style punch cards based on a CSV file, and to update them during runtime (by adding one punch at a time, then updating the file).

(it's a streamtoy for Twitch, the final display looks like this:

)

but while I can read from the file successfully (stored in the same folder as the exe,) trying to update it gets an error, and I'm not sure why!

Any help would be appreciated!

For security reasons, most modern operating systems only allow read-only access to the application folder, when installed to typical locations like "Program Files" on Windows. If you want to write data, either write it to a location the user chooses with a picker dialog, or use the app data folder (i.e. picker tag "<current-app-data>").