I managed to use Smart Shirt Designer to create kits for the club and i have done all things necessary to make it possible in game. the club i am replacing is manchester united with ID=680. If i play the normal career mode, the logo and kits are changed already, but when i play create a club they wont let me change them. They replace it with default icons from fm 17. How to use these customs? thanks!
Might be something hardcoded into the game about what they use for Create-a-Club compared to the code used for the Career mode.
Working for a software company this sort of oddity is a real possibility.