read txt.gz file

With this code you are selecting a single "cell" if you want to select the entire third column use

A <- Data[, 3]

If you need more specific help please provide a minimal REPRoducible EXample (reprex). A reprex makes it much easier for others to understand your issue and figure out how to help.

If you've never heard of a reprex before, you might want to start by reading this FAQ: