Mac Air (2015) keeps shutting off immediately after being unplugged from charger

I found this article 'About Mac notebook batteries' on Apple's website, click on the link to go to the article: About Mac notebook batteries

In the report you submitted (in the first block) I read this:

Condition: Replace Now

That's probably not good, and I think you'll have to replace the battery.

Yes, you need to replace it. I would definitely take it to an Apple Store or other Apple-authorised Repair Shop. A new battery for $130 will make your MBA 'as good as new', and restore its useful life for another 4 years.

At the very least, you could sell it on eBay, making a feature of the brand new battery.

Batteries can and do fail, and there's often no predicting it or trying to avoid it with behaviour.