Re-installing The Sims Online should fix the issue, make sure that the directory that it is trying to access exists in the first place. (make sure that the tuning.dat file is present, the whole installation of TSO needs to be in the correct path that it installed; logic)
File trying to be accessed: C:/Program Files/Maxis/The Sims Online/TSOClient/tuning.dat
Directory being accessed: C:/Program Files/Maxis/The Sims Online/TSOClient/