The reason you received the error is that the Fixed Rate field is empty when asset records were created. This field is hidden in the asset but gets populated when the Run Server SuiteScript and Trigger Workflows is enabled during the CSV Import.
1. Create a Saved Search for FAM - Asset
-Navigate to Lists > Search > Saved Searches > New
-Choose FAM - Asset
-Criteria: Fixed Rate is empty
2. Run Saved Search and click on Export - CSV
3. Edit CSV so that only 1 column remains (Internal ID)
4. Enable the CSV Import preference
-Go to Setup > Import/Export > CSV Import Preferences
-Check Run Server SuiteScript and Trigger Workflows
5. Update FAM - Asset records using CSV file created
-Navigate to Setup > Import/Export > Import CSV Records
-Import Type: Custom Records
-Record Type: FAM - Asset
-Click Select, choose the CSV file created in step 3
-Data Handling: Update
-Map Internal ID to Internal ID
6. After CSV Import runs successfully, run saved search again to make sure that no assets have empty Fixed Rate.
Perform Asset Depreciation
1. Navigate to Fixed Assets > Transactions > Asset Depreciation
2. Set the values for Asset Type(s), Subsidiary(s), and Depreciation Period
3. Click Depreciate Assets
Note: The CSV Import preference may be disabled after the import.