importdata() can return any of three different data types, depending on the content of the file.
If the file is text and has at least one header line or one comment anywhere in the file, then importdata() will return a struct with fields textdata and data with the textdata field containing the text that was read, and with data containing the numeric array.
If that is what happened then you would be trying to perform fft() on a struct rather than on the numeric data.
We recommend against using importdata() because it is too unreliable that way, too many cases you have to think to test for afterwards. Use readmatrix() or readtable() instead.
