OK, I just succeeded making the multichannel plugin by changing the PROFILE and also the RUN from Debug to Multichannel Release and it crashes the DAW, Reaper and Logic.
Here is the crash report:
Thread 10 Crashed:: com.apple.audio.IOThread.client
0 libsystem_platform.dylib 0x7ff80adcaacc _platform_memmove$VARIANT$Haswell + 268
1 HISE 16 Out 0x15af5fda3 hise::MainController::processBlockCommon(juce::AudioBuffer<float>&, juce::MidiBuffer&) + 3955
2 HISE 16 Out 0x15ae295aa void juce::JuceVST3Component::processAudio<float>(Steinberg::Vst::ProcessData&, juce::Array<float*, juce::DummyCriticalSection, 0>&) + 2538
3 HISE 16 Out 0x15ae22b72 juce::JuceVST3Component::process(Steinberg::Vst::ProcessData&) + 530
4 REAPER 0x10f92ed11 void VST_HostedPlugin::VST3_Process<float>(float**, float**, int) + 2897
5 REAPER 0x10f74eeb4 VST_HostedPlugin::ProcessSamples(int, double*, int, int, int, double, midi_List*, bool*, double, double, double, bool, bool, int) + 9172
6 REAPER 0x10f761772 FxDsp::processFxDsp(int, double*, int, int, int, int, double, midi_List*, double, bool, double, double, double, double, int) + 2546
7 REAPER 0x10f78e734 FxChain::ProcessChainDsp(FxDsp*, int&, int, int, double*, int, int, midi_List*, double, double, double, double, double, int, double, int, bool, bool, int&, int&, int) + 564
8 REAPER 0x10f78d4a3 FxChain::ProcessChain(int, double*, int, int, int, int, double, midi_List*, double, bool, int) + 1491
9 REAPER 0x10f4ad426 MediaTrack::RenderSamples_nocache(double, long long, double*, int, int, double, MediaTrack* const*, int, int*, bool, int, int, bool*, SyncSMP_Context*) + 27670
10 REAPER 0x10f4a0096 MediaTrack::RenderSamples(double, long long, int, Track_RS_Output*, int, double, MediaTrack* const*, int, midi_List*, int, int, int, int, int, MediaTrack::Track_SendRec*, bool*, SyncSMP_Context*) + 1702
11 REAPER 0x10f4aa063 MediaTrack::RenderSamples_nocache(double, long long, double*, int, int, double, MediaTrack* const*, int, int*, bool, int, int, bool*, SyncSMP_Context*) + 14419
12 REAPER 0x10f529fa2 ProcessProject(ReaProject*, int, int) + 2626
13 REAPER 0x10f528a2d audiostream_onsamples(double**, int, double**, int, int, int) + 4893
14 REAPER 0x10f5a4f8e audioStreamer_CoreAudio::onsamples(AudioBufferList const*, AudioBufferList*) + 574
15 REAPER 0x10f5a5a1f caIOproc(unsigned int, AudioTimeStamp const*, AudioBufferList const*, AudioTimeStamp const*, AudioBufferList*, AudioTimeStamp const*, void*) + 95
16 CoreAudio 0x7ff80cf3950c HALC_ProxyIOContext::IOWorkLoop() + 7734
17 CoreAudio 0x7ff80cf37041 invocation function for block in HALC_ProxyIOContext::HALC_ProxyIOContext(unsigned int, unsigned int) + 80
18 CoreAudio 0x7ff80d0a693e HALB_IOThread::Entry(void*) + 72
19 libsystem_pthread.dylib 0x7ff80ada01d3 _pthread_start + 125
20 libsystem_pthread.dylib 0x7ff80ad9bbd3 thread_start + 15
Then I manually changed the channel count in Reaper from 2 to 32 and the plugin loaded normally, which is usually not needed as Reaper does this automatically.
Just to let others know if they have the same problem with compiling the multichannel plugin.