System is activating availability control -> see long text

What causes this issue?

Availability control will be activated when you next save or check
data.
Availability control is active:
if it is so stated in availability control information (choose Extras
-> Availability control -> Info)
if you can access the "Assigned" view in the budgeting function. The
assigned value comprises actual and commitment values.
in hierarchically structured objects, such as those in Funds Management
or the Project System, if you can access the reference by choosing
Extras - > Availability control. The reference displays the following:
assigned budget
distributable budget (=distributed budget minus assigned budget)
the higher-level, budget-carrying object for an object which does not
have budget. Assignments (actual and commitments) for an object and
all the lower-level objects without budget are checked against this
"controlling element".

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.