What causes this issue?

You have changed a depreciation key in the asset master record. There
was a variable proportion amount specified in this key in the asset
master record (on the detail screen of the individual depreciation
area).
The new depreciation key &V2& is for unit-of-production depreciation and
depends on a number of units being entered. Therefore, the new key can
<ZH>not</> be used with a variable proportion amount, since it is
already defined for a proportion of 100 percent.

System Response

The proportional amount of &V1& was deleted.

How to fix this error?

The following options are available:
If you want to continue using the proportional amount, then use a
different depreciation key and enter the previous variable depreciation
amount again on the detail screen of the affected depreciation area.
If you want to use the unit-of-production-dependent depreciation key
&V2&, then you can ignore this message and continue by hitting ENTER.

Error message extract from SAP system. Copyright SAP SE.