Plugininterfaces/vst2.x/vstfxstore.h not found

Audio Plugins
2

Have a look at the JUCE 5.4 release notes, particularly the breaking changes section:

Change

The VST2 SDK embedded within JUCE has been removed.

Possible Issues

  1. Building or hosting VST2 plug-ins requires header files from the VST2 SDK,
    which is no longer part of JUCE.
  2. Building a VST2-compatible VST3 plug-in (the previous default behaviour in
    JUCE) requires header files from the VST2 SDK, which is no longer part of
    JUCE. When a new JUCE plug-in project is created the option
    JUCE_VST3_CAN_REPLACE_VST2 will be set to zero.

Workaround

  1. The VST2 SDK can be obtained from the vstsdk3610_11_06_2018_build_37 (or
    older) VST3 SDK or JUCE version 5.3.2. You should put the VST2 SDK in your
    header search paths or use the “VST (Legacy) SDK Folder” fields in the
    Projucer.
  2. For new plug-in projects where you will be releasing both a VST2 and VST3
    version, and you want the VST3 plug-in to replace the VST2 plug-in in
    hosts that support it, then you should enable the JUCE_VST3_CAN_REPLACE_VST2
    option.

Rationale
Distributing VST2 plug-ins requires a VST2 license from Steinberg. Following
Steinberg’s removal of the VST2 SDK from their public SDKs we are also removing
the VST2 SDK from the JUCE codebase.

This thread goes into more detail:

You shouldn’t need a separate VST3 SDK if you are not modifying it, as JUCE contains an embedded VST3 SDK.

If you have already released a plug-in then you will need the VST2 SDK files. If you have not released a plug-in then set JUCE_VST3_CAN_REPLACE_VST2=0.