I want to know why I can't use IntelliSense in snippet? Is there a switch that can be controlled?
Try changing this setting to false:
false
Editor › Suggest: Snippets Prevent Quick Suggestions Control whether> an active snippet prevents quick suggestions.
Editor › Suggest: Snippets Prevent Quick Suggestions
Control whether> an active snippet prevents quick suggestions.
"editor.suggest.snippetsPreventQuickSuggestions": true
true is the default, sounds like you want false.
true