i haven't been able to update pages since version 5.1

I think your feeling is correct—and it'll be the new computer as well as the newer OS.

Apple lists the hardware requirements for Mojave in the article linked below. Your MB air is too old by one year.

macOS Mojave - Technical Specifications

Age requirements are similar, but more complicated for Catalina, according to this article from everymac.com:

macOS Catalina Compatible Macs and System Requirements ...

Depending what other applications you are using, you may want to hold on to the MacBook air as well. The big change with Catalina is that it requires 64bit applications, and will not run the (older) 32bit versions. Some of your software is unlikely to survive the transition.

Regards,

Barry

Hi Roxanna,

What version of OS X/macOS is running on your computer? (Apple menu > About this Mac)

What model is your Mac? (Information is below the OS version number in the same window as opened above.

Numbers 6.2 requires at least macOS 10.14 (Mojave) or macOS 10.15 (Catalina).

If your version number is less that 10.14, you will need to upgrade the OS before installing the Numbers upgrade.

There are also minimum hardware requirements.

Regards,

Barry