Your method call does not align with the method signature provided.
// Method signature
static createUser(identity, FirstName, LastName, FiscalCode /*, Password*/)
// Incorrect Method call
User.createUser({identity: identity, FirstName: this.props.FirstName, LastName: this.props.LastName, FiscalCode: this.props.FiscalCode})
// Correct Method call
User.createUser(identity, this.props.FirstName, this.props.LastName, this.props.FiscalCode)
Furthermore, in the if condition, you are referencing class properties rather than state.
// Incorrect
utility.isEmpty(this.Password) ||
utility.isEmpty(this.PasswordRepeat)
// Corrected
utility.isEmpty(this.state.Password) ||
utility.isEmpty(this.state.PasswordRepeat)