First of all, please surround your code by three backticks like ``` before and after the code block to make the forum render it nicely with code highlighting.
That being said, your problem is that juce::dsp::FilterDesign::designIIRLowpassHighOrderButterworthMethod returns an array of coefficient structs and not a single set of coefficients. The intention of this signature is that this function is intended to parametrize a serial chain of multiple IIR instances and returns coefficients for each of them.
This thread discusses a similar problem and presents a possible solution.