I think this is a bug in how the function tries to find out the size of the SPC file. You will see this error if the file is located in a folder that was added to MATLAB's path. The function fopen that opens the file is able to locate files on the MATLAB path, but the function dir that gives file information does not find files on the MATLAB path.
One solution is to change your current working directory to the folder that contains the SPC file before running the command. Another solution would be to specify the relative or absolute path to the file when you call tgspcread. For example, if you're using Linux, and the file is in the /tmp folder, you could update your line of code to tgspcread('/tmp/new.spc').