Once you have your text, select it and then do Path->Object to Path. I then select the text and do a Object->Ungroup. This will make each letter selectable. I do this so I can easily grab the letters that I want to union. In the example below the D is separated from the other letters and has nothing to union with.
When a union (or difference, or intersection, or any thing else doesn’t seem to be working, look at the bottom of your Inkscape window:
Rather than provide an in-your-face message that you’re doing something wrong, it quietly says what is wrong at the bottom. It is where it says “Alt: click to select under…” in the image above. If you are trying to union and it says something like group of six objects, the problem is you need to Ungroup. Union and the other boolean operations primarily only work on paths.