When you get a problem like this it is because the variable is not of a struct as you expect. In general you can add a line with something like this:
just after your importdata-call. That will tell you what the variable is.
Another thing to do is to set a debug-stop like this:
and rerun your script/function, then matlab will stop at the point of error, giving you access to the workspace where the error occured, so that you can have a look at the variables with interactive keyboard commands, you can even step up and down in the call-stack to
investigate how you ended up where you ended up. (This is a necessary skill to learn, not a funny one to do, but as someone said "to err is human"...)