That error means your list of data is not a list of {x,y,z} values like:

mydata = {{1, 1, 1}, {2, 2, 3}, {3, 3, 3}, {4, 4, 3}};

FindFit[mydata,

a*x + b*y + c*x*y + e*x^2 + e*y^2, {a, b, c, e, d}, {x, y}]

First make sure your data is in the correct format: Test the code first with an explicit definition of the data like I did above. Then get the file reading part to read in the data as the explicit format you now know works. If you want to fit {x,y} pairs in the form of a function y=f(x), then you'll have to specify the expression for the function and just one variable {x}. There are however other functions that may suit your need like Fit, Interpolating Function, etc. Check the help.