Curious to learn if you've gotten the MacBook Air to charge up to over 75% before
you've tried to run it; at anytime recently? Or if you've ran it lowly on battery often?
What happens if you let it charge more than 24 hours with the Mac shut down?
Then try start up in Safe mode, and attempt troubleshooting. Various power reset
and other resets may be helpful; at the point where the battery power is sustained
but is not necessary. The power system with worn items, may need new repair parts.
Battery, power adapter, & I/O Board (internal combo/charge/power circuit) may have
defects that require replacements; these are three prime areas that can affect Mac.
The AC power adapter, probably MagSafe design, may have issues with connection
pins; if these are not clean and making connections, the battery won't fully charge.
• About Mac notebook batteries - Apple Support
https://support.apple.com/en-us/HT204054
• Apple Portables: Troubleshooting MagSafe adapters - Apple Support
https://support.apple.com/en-us/HT203207
Should power adapter test OK, then replace battery; external things to check/test.
An idea would be to troubleshoot; depending on the cause/cost, and get estimates.
Take care & happy trails!🌞🐌