hey @ppines - I think there might be a couple of things going on with your pipeline as it stands right now. Off the bat I can see what I think is a missing closing parentheses in your code. I believe you need a ) after the second condition for filter() followed by the pipe operator:
Lab_Ash_Calculated <- Lab_Ash_Data %>%
filter(InitialSampleTinWeight ==0 &
FinalSampleTinWeight ==0) %>%
mutate(Ash_Data=Ash_Data$FinalWeight)/ InitialWeight)
I'm assuming that 0 values in InitialSampleTinWeight and FinalSampleTinWeight are what you refer to as "missing"? If so, you're actually subsetting all of the missing data into a single object instead of subsetting all of the valid data. Using != will reverse this behavior.
The mutate() call also mixes in some base R "dollar sign" syntax in it that you'll want to clean up. You are currently creating a new variable named Ash_Data and then making reference to a data frame also called Ash_Data. There is also a closing ) in the midst of your mutate() call that will cause an error as well.
I suspect this is closer to what you ultimately want:
Lab_Ash_Calculated <- Lab_Ash_Data %>%
filter(InitialSampleTinWeight != 0 &
FinalSampleTinWeight != 0) %>%
mutate(Ash_Ratio = FinalWeight/ InitialWeight)
Hope these observations are helpful!