For your security Mac OS applications distributed via the Mac App Store are required to operate within a sandbox, which prevents them from accessing files which are not part of the app without requesting permission to do so.
Mosiac needs to be able to query the size of windows from other applications and set their size and position. Mosaic achieves this using Accessibilty features of macOS, but for your security we require you to grant permission fdor Mosaic to use this..
When you tap the "Grant Access..." button you will be presented with the Security & Privacy system preferences panel, with the Privacy tab selected and the Accessibilty options highlighted in the list. If the padlock icon shows locked you must click this and enter your administrator password to allow you to make changes. Once this is done you can simply tick the checkbox for Mosaic in the list.