Puco, please notice the posts above yours from me Fillister, dated 13 FEB and in particular TIG's reply.
I was receiving the same "return nil" message and TIGs helpful suggestion resolved the error message associated with SketchyFFD.

After the return nil ^-----> . in your post, I am not sure you noticed, the error messages are no longer associated with SketchyFFD.

Six lines refer to scf-rbztools
c:/users/pucon/.../plugins/sketchucation/scf-rbztools.rbs:

Four lines refer to scf_dialog
c:/users/pucon/.../plugins/sketchucation/scf_dialog.rbs:

Sadly I am unable to speak to those issues.

TIG my apologies.
In my excitement at having the return nil error so easily fixed, I rushed in to try SketchyFFD carelessly.
I can now confirm there seems to be nothing wrong as far as I can tell with SketchyFFD running in SketchUp 2019 on Windows 10.

Thank you for the quick reply TIG.
Your suggestion took care of the error message I was seeing. No error messages at load time now, but does not actually load. I saw your previous post:

Please try running it [following the earlier guidance] with the Ruby Console open and post any error messages you might get...

Wading through the 13 pages of posts associated with SketchyFFD to try and find the guidance. I will do my best to get you some more clues.

May have to learn Ruby.

This is the message I am seeing in conjunction with 2019

Error: #<SyntaxError: C:/.../SketchUp/SketchUp 2019/SketchUp/Plugins/SketchyFFD.rb:32: Invalid return in class/module body
return nil
^~~~~~>

And a sincere thank you TIG, as this was not your plugin originally

Very sorry to say SketchUp 2019 appears to have caused a conflict. These messages did not appear with 2018.

Error Loading File C:/.../SketchUp/SketchUp 2019/SketchUp/Plugins/ExtrudeTools/ExtrudeToolsLoader.rb
Error: #<SyntaxError: C:/.../SketchUp/SketchUp 2019/SketchUp/Plugins/ExtrudeTools/ExtrudeToolsLoader.rb:33: Invalid return in class/module body
return nil if noPlugin ### we've go...
^~~~~~>
C:/Program Files/SketchUp/SketchUp 2019/Tools/extensions.rb:197:in require' C:/Program Files/SketchUp/SketchUp 2019/Tools/extensions.rb:197:in load'
C:/.../SketchUp/SketchUp 2019/SketchUp/Plugins/ExtrudeTools.rb:67:in register_extension' C:/.../SketchUp/SketchUp 2019/SketchUp/Plugins/ExtrudeTools.rb:67:in module:ExtrudeTools'
C:/.../SketchUp/SketchUp 2019/SketchUp/Plugins/ExtrudeTools.rb:24:in `<top (required)>'

Other TIG plugins seem to be fine: Lattice Maker, Mirror, Split tools, TIG Weld

Your time and abilities are so valuable. I hope you can find some way to look into this.