I would recommend you use readtable instead of importdata. importdata may not return what you expected if the file format change. However, it is not the problem here.
M.data is indeed a structure ... until you stomp on it and replace it with a matrix:
From this point onward, you've replaced the original M so of course, M.data.damping no longer work.
Morale of the story: Use better variable names, ones that are not ambiguous, so you know what they actually contain. I would recommend using complete words with no abbreviation, e.g. importeddata instead of M, mass instead of the other M, stiffness, damping, etc.
Also note, that
when Y is a 2D matrix is just a more complicated and confusing way of writing: