Gap &2 to &3 exists before assortment declaration &1

What causes this issue?

There must not be any gaps in the assortment declaration history. If a
gap has occurred as the result of an incorrect deletion or a direct
deletion in the database, all declarations in the period after the gap
must be canceled. The next declaration must then begin with the start
date of the period containing gaps. It is not possible only to declare
the period with gaps since the subsequent declarations should not exist
at all on account of the gap.
The described situation is an exception.

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?

The following example illustrates how to proceed.
The following declarations exist:
01.01.2001 to 31.03.2001, 01.04.2001 to 30.06.2001,
01.07.2001 to 30.09.2001 and 01.01.2002 to 31.03.2002
A gap occurred from 01.10.2001 to 31.12.2001. Consequently, all
declarations subsequent to 31.12.2001 should be canceled.

Error message extract from SAP system. Copyright SAP SE.