You're trying to vertically concatenate arrays with different number of columns (2, 1, 1, and 20), which indeed is not possible and makes no sense.
You wouldn't be able to horizontally concatenate them either since they also have a different number of rows (1, 0, 0, and 1).
If you really need to concatenate them into something you 'll have to reshape them to at least one common dimension which is likely a vector:
sort([a(:); b(:); c(:); d(:)])
whether or not it makes sense to reshape them into vectors, only you knows.
On the other hand, if you should have been able to vertically concatenate the vectors in the first place, then something has gone wrong in the code before that and you'll have to explain how these array came to be if you want more help.