Unknown error with my data

[beginner984]

Hi,

I am permanently getting this error

sigs.input <- mut.to.sigs.input(mut.ref = a,

•                             sample.id = "Sample", 
  

•                             chr = "chr", 
  

•                             pos = "pos", 
  

•                             ref = "ref", 
  

•                             alt = "alt")
  

Error in [.data.frame(mut.full, , c(sample.id, chr, pos, ref, alt)) :
undefined columns selected

This is link to my data
https://www.dropbox.com/s/1rf9cps97znae03/myfile.txt?dl=0
Could I please ask to have a look to see why I am failing?