You can't post depreciation manually as we have a dedicated section on Fixed Assets where you can run depreciation.

We have checked and found that the asset has been added with a consolidated figure, hence the problem. To run the depreciation or disposal or sale of an individual asset, you need to add each asset individually instead of a consolidated figure which will allow you to take the necessary action on each asset as desired. 

To do so, you may edit the opening balances and add the assets individually and once done you may explain the assets and run the depreciation as desired.