This is definitely an error from BlibBuild. It is instructing you to provide the modifications.xml file used with your MaxQuant search, which must be the source of the modification you are showing in your MaxQuant screenshot. It seems that our default modifications.xml must not have this modification. I just checked the modifications.xml that our build process puts in the same directory as Skyline.exe and indeed it does not contain this modification.
It is date stamped 8/14/2018. Maybe it is time for us to update our default modifications.xml. Can you locate yours and let us know how recent it is?
The quickest solution for you, however, is to follow the instructions of the error report and place your modifications.xml in the same folder as your msms.txt file.
Hope this helps. Thanks for the quick response.