Error in eval(predvars, data, env) : object 'IQPre' not found
The error message here is telling you that in the line mod.full = lm(data=data, IQ8 ~ IQPre + Treatment), R can't find the object IQPre.
Since you set up that linear regression function call to refer to the table "data", My best guess is that table doesn't have a column named IQPre. Double check it's there, and how it's spelled (R is case sensitive).