Mercury expects certain information to be present within an input file - if certain key data are not present, then Mercury may fail to read in the file.
In the case of CIFs, the spacegroup must be specified with at least one of the following must be supplied:
Additionally, all of the following CIF data names are required by Mercury:
We would also recommend the use of the CCDC's free CIF editor enCIFer to help correct CIFs for input into Mercury. It can be downloaded here.