I did successfully my 2020 tax returns on my desktop but when I tried to open (transfer) the file on my new laptop, it told me it couldn't transfer the return but gave no reason. what do I do?

A 2019 tax data file can only be opened using the 2019 software, it cannot be transferred to the 2019 program

With the 2019 program started, click on the File menu at the top left of the program screen, click on Open Tax Return.  Browse to where you stored the tax data file.