If it shows up in your Windows account, and you are still seeing the watermark, please try the following steps:
Try to install PicCollage from another Win10 device and then log in to your windows account through the app to see if you still see the watermark? If the watermark is gone, then you probably will need to delete PicCollage from your current device (but be sure to backup your collages first if you want to keep them), and then reinstall the app. If it is still not showing up, you will need to contact Windows Support for a refund or try to request a refund directly through your account, with instructions here: https://www.pcgamesn.com/microsoft/microsoft-store-refund-policy-windows-10
Let us know if these solutions still do not work for you!