Why might the "add field" button be disabled?

[#1]

I'm testing Qlik Sense 3.0, and my first need is to get it to recognize my dates as such (they're in ISO format, I don't know why it can't figure this out on its own, but anyway).  So I go to Data Manager, select my data source, click the edit button, and see "Add field" in the top-right.  Trouble is, it's disabled, with no indication as to why.

Why might the "add field" button be disabled?

I can rename fields, so the data source is not completely locked down.  The source was two .CSV files, automatically concatenated; can one not add calculated fields to a concatenated data source?  I'm not using a manual load script.

[jharke]

I have exactly the same: I cannot add a (calculated) field: the option is greyed out. After also concatenating a whole table (automatically). Why is that and how to enable the option so that I do not have to write in script.

(To not open a new thread, I reply this old one)