How can I get rid of partition on MacBook Air?

Your picture clearly shows your MBAir only has a 250GB SSD installed. If the seller advertised a 500GB SSD, then they misrepresented this laptop to you. In fact according to MacTracker, the largest SSD Apple used in the MBAir 2015 model was the 250GB SSD....there was no 500GB option from Apple.

However, you do have an odd APFS volume there named "Spare HD" that is 4.3GB in size. That can easily be deleted by using Disk Utility if it was from the previous owner.

Welcome to Apple!

To get an accurate picture of how your Mac's internal drive is partitioned, please post the results of the following command in the Terminal app. You can find this app at: /Applications/Utilities.

• diskutil list internal

1. Run the Terminal app.
2. Enter the above command, and then, press the <enter> key.
3. Post an image of the results.
4. To close the Terminal, enter exit at the prompt, and then, press the <enter> key. Then quit the Terminal app.

The results should look something like the following:

Recent versions of macOS have a complicated disk structure intended to harden macOS against attack … and they also take advantage of APFS features that let several volumes share a common pool of free space.

Here, if I click on any of the four lines following "Container disk3", I'm told that there is 222.52 GB of free space. But that does not mean that my Mac has a drive that holds in excess of 890.08 GB. That 222.52 GB is free space which the Mac can dynamically allocate to "Macintosh HD" or "Data" (two separate volumes that each contain a portion of what the Finder displays as "Macintosh HD") as needed.

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