Two things to check:
1. I've found that Xcode updates will often remove the simulators for the previous version of the iOS SDK.
Can you try editing your
"devicesdk": "iOS 11.0"
2. Are you still on platform version
v2.5.x? Apple shipped some breakage with Xcode 9.0 that can cause issues with some simulator configurations. Upgrading to Forge
v2.6.2 will ensure full compatibility with Xcode 9.0.
Otherwise, over the years we've also found that Xcode doesn't always do a good job of initializing the new simulator definitions after an upgrade.
On some of my machines I've often found I need to run Xcode a few times, do a reboot (or two!) and maybe even launch a native app on the Simulator from Xcode itself before the definitions become accessible to anything outside of it.
You can also try a hard reset of the Xcode links:
sudo xcode-select --reset
Another thing to check for sanity is to run the command Forge uses to enumerate the available simulators and make sure the combination you requested is available:
xcrun simctl list