Update iPhone 7 to iOS 15.7.3

You do not have enough storage space on your iPhone to download the update.

Go to: Settings → General → iPhone Storage

Find some apps you don't need and choose to either delete or offload them.

You can find more useful information here:

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

Your iPhone ran out of space, you need to free some space before you're able to install the update.

To check what is taking space, review this article How to check the storage on your iPhone and iPad - Apple Support and here's some helpful tips to manage your iPhone storage Manage your photo and video storage - Apple Support

You also need to get that No SIM issue fixed first if you plan to update wirelessly.