First enable CoreGui.
Follow these steps of how to enable it!
Press Alt+S or Go to Files then click settings
Click that then click “settings”
Go to explorer then click “Show Core GUI in Explorer while Playing”
Save it and restart studio.
Edit the ChatBubble in CoreGui, Go to line 306.
Add this at the line listed above;
billboardGui.AlwaysOnTop = true
AlwaysOnTop means it override parts and UI.
Read this for more information! 
https://developer.roblox.com/en-us/api-reference/property/BillboardGui/AlwaysOnTop
Also, I advise you to read;
https://developer.roblox.com/en-us/api-reference/class/BillboardGui