my iphone 11 saids iphone unavailable, how could i fix this?

Follow the instructions in the Apple Support document that is linked above.....and below:

If you forgot your iPhone passcode

If your iPhone says "iPhone Unavailable," it means that your phone is not able to connect to the cellular network. There are a few things you can try to fix this: First, make sure that your phone is in an area with good cellular reception. If you're in a basement or a rural area, you may not have enough bars to connect to the network. Second, try restarting your phone. This will sometimes clear up any connection issues. Third, if you have an iPhone 6 or later, you can try resetting your network settings. To do this, go to Settings > General > Reset > Reset Network Settings. This will clear all your saved Wi-Fi passwords and other network settings. If none of these solutions work, you may need to contact your carrier to see if there are any outages in your area. - Hammy.