Hello Georg!

InnoDB is a storage engine.
Each table in MySQL database has property Engine, you can find it in the Property Inspector.
Different engines exist - MyISAM, InnoDB, Memory, etc.. All engines have different capabilities.
InnoDB supports the creation of foreign key links, MyISAM doesn't.

So you can switch Engine from (I suppose) MyISAM to InnoDB in the Property Inspector.
Note, it is recommended to create a backup before doing such operation to avoid any data loss.

Sergey,

maybe we need to make msg more expressive? Something as

" You using now MyISAM engine, which not supports Foreign Key. To use them you need switch to InnoDB engine"

Hello Sergey,

thank you for your help!

After some research it was quite easy to solve the problem. I made a backup of the database. Then I had to edit the sql file and execute only one find and replace command: replacing each "ENGINE = MyISAM" with "ENGINE = InnoDB". Then I restored the database with the edited backup sql file. Done. I can work with the database just like before, and in additon I can use Valentina Studio, create a diagram, create links and so on.

Best wishes
Georg