Cannot Drop Footage Into Final Cut Pro I've just bought a second-handed MacBook Pro (late 2013 - Catalina 10.15.1,) and with the deal it comes with FCP 10.4.4. 

For this version, I cannot drop the footage from the Finder to the program, I also cannot import the sound effects (with built-in ) into the timeline. The warning is "not enough disk space" even though I gave FCP full disk access and there are plenty of space in my Mac. 

I cannot update it because it was purchased by the previous owner. I don't want to have to purchase it again.
As you mentioned - you will need to free up other disk space to see if you can keep the old software running without having a license to upgrade.
Are you able to post a disk usage summary so we could help advise what you can delete or move to another disk so FCP can limp along a bit longer?
Next time you’ll know to discount the purchase price so you can fund the software you need. I don’t have any good option and know gaining experience through unmet expectations can be painful sometimes.