Pro Tools loads plug-ins in Avid's AAX format only — you can't insert a VST, VST3, or AU plug-in directly. The fix is a wrapper host: a plug-in that is AAX and loads your VST/AU plug-ins inside it, so Pro Tools sees one AAX insert while you run the plug-ins it can't load natively. Looking for native AAX instead? See the AAX plugin browser →.