This is because at the System Setting, you have opted "Warn" for Check Negative Stock.
Step 1 : Go to your setting at "Check Negative Stock" in System Setting.
2. Change the setting to "Do Not Check" to allow negative stock and the message of insufficient stock will not appear while you are doing your sales invoicing.
3. However, when you choose "Do Not Allow". This will disallow invoicing of negative stock balance.