my mac suddenely shut down when it reaches 20% level battery

Could be nearing end of battery life; be sure when you're able

run MacBook from AC power adapter. +Charge it ahead of time.

• About Mac notebook batteries - Apple Support

https://support.apple.com/en-us/HT204054

..and how to maximize battery life and lifespan..

May be a charging issue; would need more info before venturing

guess. Could be hardware failure in addition to battery age/use.

• Apple Portables: Troubleshooting MagSafe adapters - Apple

(some newer portable macs have gotten away from magsafe)

Troubleshooting by 'trial and error' may arrive at a determination.

• Reset the System Management Controller (SMC) on your Mac - apple support

One of the options depending on locality, allows user to return

the Mac to Apple repair in box they send out for this purpose.

• Apple Repair and Repair Status Check - Official Apple Support

https://support.apple.com/repair

You may have severe battery issues. Look at the battery icon at the right top corner of your screen. Look at the battery status. Send back the status.

Here is more info on Notebook batteries: https://support.apple.com/en-us/HT204054