"System" Applescripts broken by Catalina?

I have rewritten the old AppleScript that you posted. The replacement script works from the menu extras Script menu, as well as interactively if you choose to run it standalone in that manner. It checks if a folder is selected in the Finder and if true, then prompts you for the prefix/suffix string. Otherwise, it will open a folder chooser, followed by the string prompt. Based on the Prefix, or Suffix button selected, that string will be applied to every filename in the chosen folder.

You copy/paste the following AppleScript code into your open Script Editor, and then save that as AppleScript source. Then by pressing the option key, you Save As… to a script (.scpt) format. In your /Users/yourname/Library/Scripts folder, you create a new folder named Finder Items, and copy your .scpt version of the script into that folder.

When you select Script Editor : Preferences : General, use these settings:

to have this, where I named the script prefix_suffix.scpt:

The first time you use this, Catalina is going to go security in your face. Just click OK to allow the script permissions to do its thing:

Apple has reversed the order of the result list returned from the display dialog from what it was when this script was originally written in the early to mid 2000's. Consequently, and as written, the script now assigns the button Prefix, or Suffix name as the string, and not what is entered at the prompt. This is easily fixed and solves the prefix string issue, but the script logic is broken with suffixes as it appends your input string to the extension name, not to the base filename prior to the extension. This renders the filename as an unknown to the operating system.

This script could be eventually modernized to apply a prefix, or suffix to the base filename in less code. Let me see what I can do with the existing code with regard to proper suffix application.

[Update} I now have prefix and suffix strings working correctly on the base filename string, without impacting the extension name. But more testing is needed.

This functionality is now built into the operating system. Select multiple files in the finder and choose the "Rename" option:

This will bring up a batch rename dialog with several different options:

Neither Catalina, Mojave, High Sierra, nor El Capitan provide a Finder Scripts folder, or the Add to File Names.scpt script in the following path locations:

1. /Library/Scripts
2. /Users/yourname/Library/Scripts

This suggests to me that this is something you installed yourself, and/or has propagated via either migration assistant from an older operating system, a Time Machine restore, or years of just upgrades to the operating system.

Click the code <> button in the bottom toolbar here, and after you open the Add to File Names.scpt, select all of the code, and copy/paste it into the the area defined by the <>, enter an extra line after the pasted code, and then click the <> button again. This is the only way you can share code here that will not be a total mess.

It might be far more practical in Catalina to use the Finder's Rename feature from the secondary menu, as I have shown in a preceding post. It will allow you to set prefix or suffix in the desired manner on selected filenames.

Is it your expectation that you want a script that will apply your prefix or suffix string to the beginning, or end of the base filename encountered (e.g. string_filename, or filename_string), without altering the extension name? That is not how this script currently works though.

As tested here, the script determines if you have already selected a folder, or filenames in the Finder when it is run. If so, and it is a folder, then every file in its first level contents gets either the prefix, or suffix treatment. If it is one or more files, then the chosen prefix or suffix is applied to each selected file(s). If there is no Finder selection at all, then it allows you to pick the folder whose first level contents that you want to apply the prefix, or suffix string too.

The Finder Rename : Add Text panel does not apply its changes to sub-folder contents either.

Or in the context of the OP's requirement of prefix or suffix:

Yes, alot of gnashing teeth over that 5000 character limit, and sometimes Additional text can't hold it, and one must do several replies with segments of the code in the <> tool. PITA.

I have a yard to mow, so will look over the code when that is done.