@jcblum has the answer for what you want.
The answer for what happened: you used a parenthesis when you meant to use a bracket.
table(final_df$Gross_MarginDollars_Percentage( # here
final_df$Gross_MarginDollars_Percentage is (assumedly) a vector. Following it up with a parenthesis means it should be used as a function, but R notices it's not a function. Hence the error message
attempt to apply non-function
Errors and warnings can be cryptic, but they're also consistent. Now you'll know what this one means when it pops up again (and it will, for we are but humans who type with dumb fingers).