It seems that the values you are trying to add to the dataframe are of class "structure".
You need to use indices to select elements from myCoefs@Dimnames[].
Make the highlighted changes to your code and it should fix the error.
sze = length(myCoefs)
myResults <- data.frame(features = myCoefs@Dimnames[][1:sze],
featuresName = fnames[1:sze],
coefs = myCoefs[1:sze])