refurbished Macbook won't update

Try an SMC reset:

https://support.apple.com/en-us/HT201295

Try starting up in Safe Mode:

Use safe mode to isolate issues with your Mac - Apple Support

If neither of those work boot into Recovery and Reinstall Mac OS.

About macOS Recovery - Apple Support

Try Single User Mode (hold command-s keys)

https://support.apple.com/en-us/HT201573

At the prompt, enter this command:

/sbin/fsck -fy

Press the Return key.

This will check your directory and report that it was OK or repaired. Then enter this command to startup normally:

reboot

If none of those work, you have 90 days of free telephone support with your purchase (800)MYAPPLE. I suggest you use it to open a ticket. You also have a one year warranty and you can make a Genius Bar appointment for a free evaluation:

Genius Bar Reservation and Apple Support Options - Apple