Level &1 is not part of the cash management position (source symbol &2)

What causes this issue?

The cash forecast level &V1& does not belong to the cash management
position and therefore cannot be used for G/L accounts or G/L account
line items, only for customers and vendors.
(The levels are defined in the view V_T036 and assigned to a source
symbol there. This source symbol is assigned to the cash management
position or to the liquidity forecast in the view V_T039).

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.