For future reference:
I don't know what is in your TFA file. If it's XML, then the following may apply:
You specified an -asl option -asl1
This is telling JfMerge to skip one line at the start of the data file.
You've likely then caused JfMerge to try reading the TFA from somewhere other than the legitimate beginning.
Yes, the error message would then be misleading. But the error message mentions "skip length" and the -asl option is the skip length definition.