How do I find the value in the first column according to the edited cell? (Google Sheets)

Question

I'm trying to create a sheet in the same workbook that will log changes that are made on the master sheet.

Here is the 'Master List' where changes will be made:

This following information needs to be logged in the 'Logging Sheet' Screenshot Below:

'First Name (Value in First Column)','Column Header (in Row 1)' to 'Value in Current Cell' by 'Active User' on 'Time,Date', 'Cell Ref', 'Sheet Ref'.

I'm trying to find a way to be able to place the First Name + Last Name of the person the information was changed for in the first column of the logging sheet when the code is executed.

Here is the current code I'm using:

I'm also getting a

TypeError: Cannot read property "source" from undefined. (line 2, file "Code")

but the code still runs.

Here's a link to a test sheet that you can make a copy of and check out: test spreadsheet