Error: Free space for index is not sufficient

What causes this issue?

The free space check prior to converting a table found that the free
space in the target tablespace or target DBSpace of the primary index
is not enough for a successful conversion.

System Response

The conversion is not started.

How to fix this error?

Using the log, find out how much more storage space is needed in the
tablespace or DBSpace. Enlarge the tablespace or DBSpace. Then start
the conversion again.

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

Error message extract from SAP system. Copyright SAP SE.