play through the main route then the collectmode unlocks

Collection System is working. It is unlocked when you have already finished the story.

You have to know that it is necessary to buy DLC to find 100% of the items. For example, the image n*20 in the gallery is a flower bracelet that you can find with the DLC released the 22 march 2018 (Clara DLC).
You have also the free Happy birthday DLC (that is unlocked in the Extra menu when your progression is important enough). There, you can find candles on the ground in a picture.

I need to find 2 more items to have 100% (95% now). If somebody know where to find them ? Help is welcome... Or maybe it is not possible to have 100% items now??? Maybe it will be necessary to buy another DLC to have the last items...

1 / first image in the items gallery (left of the orange t-shirt)
2/ image 19 (between white/red rubban (n°18) and flowers bracelet (n°20)

Use this guide.

Thanks ! I didn't see this guide before !!! I have got all items now !