qqman | R package for creating Q | Genomics library

I have a very stupid question. It has been already asked, but none of the solutions provided seem to work with me. I am looping over a list containing different data frames, to perform an analysis and save an output file named differently for each input data frame. The name would be something like originalname_output.txt. I wrote this piece of code which seems to work fine (does all the analysis in the correct ways), but gives an error when coming to the write.table part.

Very new to R and coding in general, so having some serious issues! I have a dataframe containing 1,887,181 entries from a GWAS. I am trying to use the qqman package in order to create a manhattan plot. The issue I am having is that the data I need to create a BP column is given in a mixed string format with the chromosome number in an ID column. For the top associated SNPs (of which there are only 45 entries) I've been using as.numeric_version() in order to extract the numeric part from the ID column which works but is definitely an incorrect way to go about it, as it gives the following error, but still outputs the correct values: