The format is incorrect. In your screenshot, SLADE lists their format as "Graphic (Doom)", which is for sprites, textures, HUD stuff, et cetera. Flats should be of the "Graphic (Flat)" type (which only works with certain specific image sizes, usually being power-of-two sized with equal width and height like 32x32, 64x64, and 128x128; your 16x112 image won't work in this case).[...] I'm getting a "Flat lump xxxxxxxx data format could not be read" error, even though the format is correct. [...]
SRB2 supports using textures on floors/ceilings. If SRB2Kart supports this as well (which I don't know whether or not it does), you can get away with simply removing the "F_FLOOR" and "F_END" markers, then moving the "TX_END" marker to be just above "MAPZZ".