Cannot Drop Footage Into Final Cut Pro
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.