Please don't work on your side business on company time.
work on ...は「…に取り組む」という意味です。
「内職(副職)」は side-job や side business などと言えます。
また、「仕事中」はat work や on company time で表せます。
その他の言い方:
- Please don't work on your sideline job while you're at work.
- Please don't do any sideline work while you're at your main job.
- Please don't work on your side project while you're at work.