My turbotax can't find the data file in the same computer I was using to start my 2018 tax return last December of 2018?

What are you trying to do?  You don't Transfer a .tax2018 File.  You just OPEN it.  Go up to
File-Open

turbotax is suppose to look for previous tax files, it couldn't find the file I wanted to open, I did what you suggested and it worked. Thank you.

Thank you, problem solved

Yes it looks for Previous YEAR files at first to Transfer into 2018.  You already had a 2018 file to open.