Why is there an error when installing an ios 15.7.3 update
Why is there an error on new software update?
If your Apple mobile device won't update, read this support article --> If your iPhone or iPad won't update - Apple Support
Another, less common reason for not being able to update is if you were using a beta version of software. Refer to How to uninstall iOS or iPadOS beta software - Apple Support
If you wish to provide us with more specific information about the error message, your wifi and/or data connection strength (if using 5G) then that may help us narrow it down. Right now we have almost no information to do anything except make broad suggestions for things you can read.