"Could not insert new outlet connection: could not find any information > for the class named view controller" in Xcode.

I'm trying to make a discount calculation with Xcode.
Tist fields and buttons laid out in Storyboard and
I'd like to link the source code
Press the control key to select the text field and
Drag & drop to Assistant Editor source code
The following error appears:

Error Contents

Could not insert new outlet connection:could not find any information
for the class named view controller

What I tried
·Delete files for Finder > Library > Developer > Xcode > DerivedData > …
·Product Clean ++Shift+K
·Cache erasing $rm-rf~/Library/Caches/com.apple.dt.Xcode
·Xcode restart

I have tried the above, but it has not been resolved.
What other solutions are there?
Thank you for your cooperation.

Environment

macOS HighSierra macOS HighSierra 10.13.6
Xcode Version 9.4.1 (9F2000)