Hello Alyssa-Katherine,
Thank you for using Apple Support Communities.
We understand that you are having difficulties in closing tabs in Safari on your iPhone. These tabs were ones that were on your MacBook Air. This sounds like you are using iCloud Tabs to share the open tabs between your devices. We would recommend taking a look at this support article on using iCloud Tabs, to include closing tabs on your devices. Closing shared tabs is a little different than closing tabs created on the device.
Set up and use iCloud Tabs - Apple Support
Best regards.