Swiftkey red underline is gone!

I am using Samsung Galaxy S6 with Android 7.0, and here is the thing:

• When I use Swiftkey, the red underline is missing.

• When I switch back to Samsung keyboard, the red underline is working again!

So, what is the problem? I remember tech news that red underline is a spell checker feature included in the free netflix core Android OS to mark the misspelled words, and previously it was working with Swiftkey.

Can anyone assist me, or if you are a Swiftkey user on Android please tell me if you got the red underline or not?

Edit: spell checker option is not in the setting anymore! It has been moved to Samsung keyboard, is that a bug?

i.e. I can't do this:

Settings > system > language and input > advanced > spell checker > language > set English instead of system default.