Unable find preferable collections to assign filter
When you are unable to find suitable collections for your newly created filter tree, the reason behind it could be your configuration. You may have assigned those collections to an already existing filter tree in your store.
You can check whether they have been assigned or not by the following steps:
Note: You should check all filter trees to find the collection name if you have a long list by clicking the next button
The Shoes collection has already been assigned to the Shoe filter tree. Hence, it will not show up in the collection box for the new filter tree.