My macbook air has an X on its battery icon

Hold Option key and click on the battery icon.

What does it say about Battery Condition ?

I would suggest to do the SMC reset.

The X would mean No battery available, or the SMC is confused

• Shut down the MacBook and connect the MagSafe power adapter – let the Mac completely power down before proceeding any further
• Hold Shift+Control+Option+Power concurrently for a few seconds, then release
• Press the Power button as usual to start the MacBook

Your macbook maybe need to replace the battery. You can check status of battery by holing option key+battery icon in menu bar.

There will write condition: (and status of battery)

There is a second way too: https://support.apple.com/en-us/HT201585

Hope it helped a bit!

Have you checked About this Mac > the sytem report > power > battery? What is the health status? Is it normal? How many charge cycles? And what year is the device?