[lammps-users] help: datafile contains no improper

I doubt this has anything to do with impropers. The error
is proabably in your data file where you've specified a
parameter that is expected to be an integer, with a floating
point value. The most common place is dihedral coeffs for
d or n. If you specified them as a floating point value,
LAMMPS is telling you not to do that.