It means that the data type of Messdaten is not what function realdata expects.
If you do the following, you'll see the same error.
>> realdata('some nonsense data type')
Check for incorrect argument data type or missing argument in call to function 'realdata'.
Since realdata tries to see if the input argument is real type or not, I guess the function is implemented on some assumption about the input argument. And 'some nonsense data type' is a character vector which is definitely not one of the kind.
Please refer to Determine whether iddata is based on real-valued signals - MATLAB realdata (mathworks.com) for the details. In the document, one of the examples passes in zf to realdata. I would first try to make Messdaten to look like zf of the document's example.
