Apple stopped signing iOS 13.5 after releasing iOS 13.5.1 last week. The company was quick to stop signing iOS 13.5 as there was a zero-day security exploit in it that allowed for jailbreaking it. If you want to jailbreak your iPhone but accidentally upgraded it to iOS 13.5.1, don’t loose hope as there’s still a way for you to jailbreak it.
The zero-day security vulnerability in iOS 13.5 is also present in the iOS 13.5.5 beta 1 which Apple is still signing. So, if your iPhone is currently running iOS 13.5.1, you can upgrade it to iOS 13.5.5 beta 1. You will have to manually flash the IPSW though since Apple has released iOS 13.6 beta 2 so you will directly get the OTA update for it. iOS 13.6 beta 2 is a newer build than iOS 13.5 and it fixes the 0-day exploit. Apple will likely stop signing iOS 13.5.5 beta 1 within the next few days so you should install the iOS 13.5.5 beta 1 update on your iPhone as soon as possible.
Do note that you will be prompted from time to time to update your iPhone to the latest beta release, but if you are really keen on jailbreaking your iPhone, you will have to live with it.
Once you are done installing the iOS 13.5.5 beta 1 IPSW on your iPhone, grab the latest release of Unc0ver which has support for jailbreaking the OS and proceed to jailbreak your device.