I have edited your tags and title because your problem had nothing to do with "outstr", which is merely the name of the variable you happened to be assigning the result to.
Inputs can be combinations of single strings, strings in scalar cells, character arrays with the same number of rows, and same-sized cell arrays of strings.
So your problem is that txt{1} does not have the same number of rows as the [' Trade';buysell] expression does. This is because you are adding a header line for that portion, but you do not add a corresponding header (even an empty string) to the first part.
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