If you are required to set up a new fixed asset for the business unit which has no master data existing in the system, you need to create the master data for the asset in the system before you can create any transactions against this asset.
Procedure: Follow the steps from the menu tree.
1. Choose Accounts
2. Financial Accounting → Fixed Assets →Asset → Create → Asset
3. The window opens in Find mode. Curser defaults in the first screen field.
Or select the Transaction code: AS01
Enter the Transaction Code AS01
Understand a lot more about it from the document on Working with Fixed Assets