This depends on what is the root cause of the error.
In any case the layout Atlas_UI_Resources.Atlas_Default doesn't exist any more
1. You have deleted that layout > undo delete
2. You have delete the module Atlas_UI_Resources . In that case 2 options
- restore that module
- in case you deleted it for upgrade to atlas core. Then after download of Atlas_Core module format marketplace, rename them module to Atlas_UI_Resources, and after that rename it back to Atlas_Core.
- in case only one page has missing layout > open related page properties and select an available layout
Likely you are in 9.6.1 (or > 9.3.0) and using Atlas_Core, and you imported a module that is still using Atlas_UI_Resources. The above steps are the quickest and simplest way to solve this, not just for one page, but for every occurence of this error.
Goto the properties of the page and select an other layout. You probably will find another Atlas_Default there. Select that or any other one of the layouts you are using in your app.