Cannot update software on iPad using my Mac

You still need to free up much more free, internal data storage space on that iPad.

3 GBs is NOT enough to install most iPadOS software upgrades/updates.

ALL iOS/iPadOS devices need to maintain, at the very least, a minimum of between 3-6 GBs of free data storage space, AT ALL TIMES, to insure normal/nominal functionalities/operation and general overall performance of any iOS device running iOS/iPadOS.

If upgrading/updating from a much older iPadOS version, you need to make sure your iPad has, at least, between 6-8 GBs of free internal data storage space for downloading and installing a major iOS/iPadOS upgrade/update.

Good Luck!

What happens, exactly, when you try?

Make sure both your Mac and iPad have enough free, internal drive space to perform the iPad update.

Make sure your iPad is, at least, charged to 50% or greater to attempt this.

Also, make sure your iPad has, at least, between 6-8 GBs of free internal data storage space for downloading and installing a major iOS/iPadOS upgrade or update.

Your iPad is still at minimum storage amount and iPad performance will soon suffer, as a result.

You need to regain some free, internal data storage on that iPad.

Once your iPad gets to 2 GBs or lower, iPadOS WILL start having issues.

You should never, EVER completely fill up ANY computer or computing device full of data. Ever!