The error message is correct. There is no C3LB1 Bottom. It appears you deleted it. You could have assigned the end panel to C5L but you deleted that bottom also. What you can do is re-draw just cabinet C3L. When you re-draw it the cabinet will have a new number (probably C15L in your case). Now add the end panel to the new cabinet number. When you have the end panel you want, edit the cabinet you re-drew to delete the bottom and make whatever changes you need to.
The reason you are getting this error is because CabWriter assumes that all cabinets have a back and a bottom. An assumption that is almost always true. It needs to make this assumption for two reasons (features). It needs to assume there is always a back to allow the Auto Swap feature of Cabinet mode. It needs to assume there is a bottom for the Create A Back Panel, Insert A Drawer and Insert A Door features.
Perhaps for CabWriter 3 I can change this to expect only one part; a back, though I seem to recall ruling that out when I created the back panel tool. I do some more research.