GTFS files are meant to be read by machines, so little errors can cause the whole file to become unreadable. Depending on the kind of error you are encountering, or what tool is producing the error when reading the GTFS, several steps can be helpful to run through to ensure the GTFS files are performing as expected.
Note: this does not cover the contents of the GTFS file, such as incorrect stop times or stop locations showing on applications such as Google Maps.
When editing GTFS files using a tool like Microsoft Excel, it is important to not introduce empty rows or columns without headers. Because text files do not operate exactly the same as XLSX files, or even CSV files, extra columns and rows will cause the GTFS to fail validation.
To fix, use a text editing tool, like MS Notepad, to see if there is an extra comma in the header of the file. Similarly, you will be able to see if there are rows with no values (maybe just commas) at the bottom of the file.
When using Microsoft Excel or a similar program, make sure to use Delete, not Clear or Backspace to remove values from the file. Clear Contents, or the backspace key, leave the cell behind with no values. This gets interpreted as a valid entry (since GTFS files can contain blank values), but when it ends up applying to columns and rows, it will create issues when converting the files back into text files.
Nine out of ten times, when files are described as missing, but are clearly in the zip file, the issue is there is an extra folder in the zip file. When zipping up the files, you should zip up the actual text files themselves, not the folder they are enclosed in. Some zipping applications will create a second folder inside the main folder, which means when a consumer opens the zip file there are no text files there - just another folder.
If a zip file is renamed after it has been zipped up, this may also create the missing files errors. Ensure that unzipped folder name, an dthe zip file name, are identical. If they are different, there is a possibility the zip file was renamed. The solution is to rezip the files and name the GTFS at that time to the exact name necessary to support a static fetch link.