So yesterday I went to sleep and I saw an update. I didn't update since I couldn't be bothered, but when I woke up and turned on my iPhone it said update completed. But when I tried to activate it I get that message. I have an iPhone 7 plus and I've been using it for around 3.3 years. P.S the update I downloaded was IOS 13.3.1
Since you have an iPhone 7/Plus i immidiately suspect one of the many chip problems plauging those two models (actually the problem is with the motherboard substrate itself, I think).
If it is this one problem, you can find out by jumping to the initial "Hello" screen after the update, if you have that, and selecting the info i in a small circle in the bottom right corner. It will show you various hardware parameters of your phone, such as the serial number. It should also show the IMEI and a few others it gets from the mobile network chip (as far as I understand).
If it is unable to retrieve one or more of these parameters from the hardware, which is a common affliction of the iPhone 7 series, you will not see them in that info screen and it will not be able to activate, and you might get the error you are reporting, despite it managing to reach the activation server. The problem is rather that it doesn't manage to give the activation server the info it is requesting.
If you find that this is indeed the case, nobody is able to fix your phone but Apple, but you might be able to get a free service if you can convince them that it is the same problem essentially (which it is), as the one they have a free program for: https://support.apple.com/iphone-7-no-service
Wait For Sometime
Check your Wi-Fi / Network
If you're using a cellular data connection, connect to a reliable Wi-Fi network instead.
Fix iPhone Activation Errors using iTunes
If you still can't activate, here's how to activate your iPhone through iTunes instead:
• Make sure that you have the latest version of iTunes.
• Check that your computer has an Internet connection.
• Connect your iPhone to your computer via the USB cable. Turn it off and reboot.
• Open iTunes on your computer.
• Wait while iTunes detects and activates your iPhone:
• If iTunes shows, "Set up as New" or "Restore from Backup," it means that iTunes activated your iPhone.
• If an error in iTunes says that the SIM card isn't compatible, contact your carrier.