To solve your immediate problem, remove the first row from your explanatory variable, price and promotion.
But, be warned that running a regression on a differenced variable with explanatory variable in levels is likely to have insignificant results. Plot the variable to see why.
I would recommend that you difference the price and promotion as well, in which case, you wouldn't have to remove the first row. Or maybe you should as all 3 variables will have an NA in the first row.