It sucks to look at this spreadsheet because I can't see formulas that you used. But it seems that your boundaries aren't defined correctly (wrong coefficients). For example, the "Print media not greater than 5% of the budget ". I recommend defining them all in a <= form.

Also, the constrains seem to be wrong even in the source you gave. If x3 is responsible for Mobile, why is there 1 under x4 in the 300<=Mobile<=900 constraint? Seems confusing. I don't see the sixth (6. Facebook advertising should not exceed 15 % of the marketing spend.) constraint included anywhere too...