I went to search the same topic but it doesn't resolve my issue.
I add an other menu item in the navigation, the name of the collection I want to link my product, but it doesn't link
I have 2 other products already linked with the collection but not 3rd one.
there are no products in the collection under collection section.
Solved! Go to the solution
This is an accepted solution.
When you set up a new collection, you have to add the title, then choose Manual instead of Automatic as collection type. Also make sure that the bottom right section shows "collection type".
Then save. After this you can add the products into this collection individually or clicking on more with bulk edit.
Maybe you done it this way, just wondered.