Can you send us your Skyline document? I am not sure which modifications are on that peptide.

In Skyline you can use the menu item:
File > Share
to create a .zip file containing your Skyline document and supporting files including spectral libraries.
If that .zip file is less than 50MB you can attach it to this support request. You can upload larger files here:
https://skyline.ms/files.url

A mass error of +554.8 on a predicted m/z of 792 would correspond to an error which was approximately 0.43 Daltons.
When Skyline is looking at a spectrum and trying to decide which things in the observed spectrum correspond to which fragment ions, Skyline uses the "Ion Match Tolerance" setting at "Settings > Transition Settings > Library".

-- Nick