System 7.5.5 will only install after System 7.5.3 has been installed. If the System 7.5.5 installer produces the error you describe and System 7.5.3 is installed, check to be sure you have not renamed any system files. Two files, when renamed, have been found to cause the install issue you describe, their original final names are:
System 7.5 Update
System 7.5.2 Update
When updated to System 7.5.3, the System 7.5 Update file is for a system updated from System 7.5, while the System 7.5.2 Update file is for a system updated from System 7.5.2.