What is the easiest security vulnerability to close in a business?

Easiest? Lock the office door when the last person goes home each evening.

Here are a bunch of easy ones:

1. Don't write your passwords on paper next to your PC 2. Don't push production passwords to VC 3. Set up an e-mail spam filter 4. Follow the principle of least privilege

There's probably a bunch more and any decent LLM can give you a good starter list.