how to append model in blender 2.81
there is no folder named "collection"
please help me
This is a priority bug they are working on. It still works. You can either go to the advanced filter list to the bottom left and shift click on collections, or you can click the filter button on the right and go to blender IDs section and shift click on collections.
Unfortunately it seems like the bug is back for 2.81. Several people expressed surprise when several bug reports came in with the same or related issues, which were then all merged into one task. Hopefully they'll squash that bug soon, but apparently it's trickier than anticipated.
It seems like the bug has been more or less taken care of now in the latest build of 2.81.