Could not nudge the selection because the pixels in a group cannot be edited without merging the group.

We need to merge the group in order to nudge the selection.

1. Select the group from Layers palette (pressing Ctrl+E is going to do the job of two lines below)
2. Right-click on the group
3. Choose Merge Group

Hint: It's better to duplicate the group and hide it before merging so that we can edit layers in the group when we need them.

1. Select the group from Layers palette
2. Right-click on the group
3. Choose Duplicate Group...
4. To hide this group, click the "eye" icon next to its thumbnail