Predictive Accuracy formula in Excel or R

Recently, I have built a model and I have the output similar to this below. The output is in the Excel. I am trying to understand if prediction error can be calculated in the Excel? For example, could I write a formula in excel to determine the predictive accuracy for the table below? I was thinking to do something like this: predicted - accuracy...or (predicted - accuracy)/accuracy. or use RMSE/SSE/MSE...etc

I am not sure if these ideas are right. Thanks!

customer id      predicted_sales    actual_sales    pred_minus_actual   (predicted -     actual)/actual
1A               100                150             -50                 -0.333333333
2A               200                100             100                 1
3A               300                256             44                 0.171875
1B               100                300             -200                   -0.666666667
4B               400                390             10                 0.025641026
6B               500                502             -2                 -0.003984064