When Multi-trading account setup is possible in the new company only.Also open a new Trading account you have to select the environment setting.

Step 1) Go to Environment setting > Trending .
Step 2) Select Multi Trading Account in Trading Account Setting.
Step 3) Then after go to Master > Product > Trending Account
Step 4) Click on Add ( Alt + A)
Step 5) Then open Trading Account Master Entry
Step 6) Add information of Trading Account Name and short name
Step 7) Select the Unit 1 Sales , Purchase and Stock Unit as per trading account.
Step 8) Press Save (F2)
Step 9) Then open a new product and select a trading account during the opening of the new product.