problem with replacement screen assembly

The screen assembly for the Kindle Fire HD 8.9" consists of just a digitizer screen bonded to an LCD screen. The plastic frame is not part of this assembly. When replacing a screen assembly you generally remove the old broken assembly from the plastic frame to which it is glued, and glue in a replacement.

If you acquired a new screen assembly that was already mounted to another frame, then you could have the compatibility problem you describe, because some 8.9" HD's come with SD card slot cut-outs in the screen frame and some don't. Unless you can forego the SD card feature you could then be in trouble, because you'll never remove the new screen assembly from its frame without breaking it; the glue is just too strong. My advice is to forget the SD card feature; you'll have to buy a whole new (overpriced) screen assembly to get it back. Just reassemble the Kindle using the slot-less frame; it'll work fine for most purposes!

If you really, really need the SD card feature, then consider buying a new tablet and sell the old one on eBay-- the net cost will be the same or maybe even be cheaper than buying another screen assembly for the 8.9" unit.