Look to see if you have the fgds properly installed and then check your Game Configurations tab in the Hammer Options menu if you haven't already done so you need to restart Hammer
Yours will look different from mine, but make sure both the tf-abs.fgd and the bamf fgd are in the Game Data files, if not click the Add button then scroll to the proper fgd file to include it. These FGD files should be located in the Team Fortress 2/bin folder