You have to add juce::ParameterID{"parameterID", versionHint}
OR juce::ParameterID("parameterID", versionHint)
as the 1st argument of parameter constructor in PluginProcessor.cpp.
Change your code to
juce::AudioProcessorValueTreeState::ParameterLayout SimpleEQAudioProcessor::createParameterLayout() {
juce::AudioProcessorValueTreeState::ParameterLayout layout;
layout.add(std::make_unique<juce::AudioParameterFloat>(juce::ParameterID{"LowCutFreq", 1}, "LowCutFreq", juce::NormalisableRange<float>(20.f, 20000.f, 1.f, 1.f), 20.f));
layout.add(std::make_unique<juce::AudioParameterFloat>(juce::ParameterID{"HighCutFreq", 1}, "HighCutFreq", juce::NormalisableRange<float>(20.f, 20000.f, 1.f, 1.f), 20000.f));
layout.add(std::make_unique<juce::AudioParameterFloat>(juce::ParameterID{"PeakFreq", 1}, "PeakFreq", juce::NormalisableRange<float>(20.f, 20000.f, 1.f, 1.f), 750.f));
layout.add(std::make_unique<juce::AudioParameterFloat>(juce::ParameterID{"PeakGain", 1}, "PeakGain", juce::NormalisableRange<float>(-24.f, 24.f, 0.5f, 1.f), 0.0f));
layout.add(std::make_unique<juce::AudioParameterFloat>(juce::ParameterID{"PeakQuality", 1}, "PeakQuality", juce::NormalisableRange<float>(0.1f, 10.f, 0.05f, 1.f), 1.f));
juce::StringArray stringArray;
for (int i = 0; i < 4; ++i) {
juce::String str;
str << (12 + i*12);
str << " db/Oct";
stringArray.add(str);
}
layout.add(std::make_unique<juce::AudioParameterChoice>(juce::ParameterID{"LowCutSlope", 1}, "LowCutSlope", stringArray, 0));
layout.add(std::make_unique<juce::AudioParameterChoice>(juce::ParameterID{"HighCutSlope", 1}, "HighCutSlope", stringArray, 0));
return layout;
}