Li (s) + I2 (s) --> LiI (s)
All components of a reaction must be balanced for it to be written correctly. The questions asks for a coefficient just for LiI, but we'll also make a change to the Li. We note that iodine is diatomic, so comes only in pairs. That means we need at least 2 iodine atoms in the products. So add a coefficient of "2" for the LiI. That will balance tjhe iodines to 2 for both sides.
Although not requested, we also see that 2LiI molecules will require 2 Li atoms from the left side. So add a "2" in front of lithium:
2Li (s) + I2 (s) --> 2LiI (s)
Now the equation is fully balanced.