Welcome to the forum!
Had to look up "split monogram". If you select only the text (which you entered using Inkscape), it should Union just fine. If you're trying to select the entire split monogram, who knows how that file was made?
As Moini indicated, they may be grouped. Since they are SVG files, I would not expect the split monogram to be a raster image. But theoretically, could be. Or the vector paths could be on top of raster images (so that if you used Select All, the raster would have been selected).
Echoing Moini re status bar or showing us the SVG file.