Could not find module for target 'x86_64-apple-ios-simulator'

To solve this issue I had to create a fat library of my custom framework again using xcode 11 tools.

To do that I did the following:

1) Build YourCustomFramework target for iOS simulator and extract framework from products folder on your desktop.

Xcode⁩ ▸ ⁨DerivedData⁩ ▸ ⁨Your Project ▸ ⁨Build⁩ ▸ ⁨Products⁩ ▸ ⁨Release-iphonesimulator

2) Build YourCustomFramework target for Generic iOS Device and extract framework from products folder on your desktop.

Xcode⁩ ▸ ⁨DerivedData⁩ ▸ ⁨Your Project ▸ ⁨Build⁩ ▸ ⁨Products⁩ ▸ ⁨Release-iphoneos⁩

3) Rename the simulator generated framework to YourCustomFramework-sim.framework so that it is distinguishable later.

4) Use the lipo command to combine both binaries into a single fat binary file. (cd to your desktop or wherever your custom framework file is located)

$lipo -create ./YourCustomFramework-sim.framework/YourCustomFramework ./YourCustomFramework.framework/YourCustomFramework -output ./YourCustomFramework

5) Copy YourCustomFramework binary file created in above step and replace it with the binary in YourCustomFramework.framework folder.

6) From folder

YourCustomFramework-sim.framework/Modules/YourCustomFramework.swiftmodule/

copy all of the modules and paste them to

YourCustomFramework.framework/Modules/YourCustomFramework.swiftmodule/

This should solve your issue.

Swift 5.0-5.1, Xcode 11
Open Xcode, <your project>, Build Settings, Build Active Architecture Only and change to <NO> for Debug and Release. Architectures set/leave in Standard Architecture -$(ARCHS_STANDARD), important is next step:
Valid Architecture: armv7, armv7s, arm64, arm64e, ADD here x86_64 and if you need add i386 for Debug and Release. (String: armv7, armv7s, arm64, arm64e, x86_64)

Choose any simulator in your simulator list and BUILT IT. DONE. 

I hope it is works for you.

Description of Architecture:

armv64: iPhoneX, iPhone 5s-8, iPad Air — iPad Pro

armv7 : iPhone3Gs-5c, iPad WIFI(4th gen)

armv6 : iPhone — iPhone3G

-the above if for real devices

i386 : 32-bit simulator

x86_64 : 64-bit simulator

I have added two architectures i386 and x86_64. And set Yes to "Build Active Architecture Only". It works for me to build on every simulator devices.

For the XCode-12 Valid Architectures has been moved under User-Defined option at the very bottom as VALID_ARCHS