You have a .7 instead of a .7f. I also opened it in write text mode. Try this:
clc;
x=-1:0.1:1;
a=1+(x.^2)+((x.^4)/2);
b=exp(x.^2);
c=abs(b-a);
y=[x a b c];
fid=fopen('Taylor.exp','wt');
fprintf(fid,'X Approx.Values exp(x.^2) Absolute Error\n');
fprintf(fid, '________________________________________________________________________________________________\n');
fprintf(fid, '\n%.1f\t\t%.7f\t\t%.7f\t\t%.7f\n', y);
fclose(fid);
type 'Taylor.exp'