What it proposes is that you check the data in the
acc_trans table for NULL values in the 'amount' column. A way
to get rid of it (if it's not referenced from anywhere) would
be to run the query "DELETE FROM acc_trans WHERE amount IS
NULL". By "not being referenced from anywhere", I mean that
the rows in acc_trans can themselves reference "invoice" rows
or be referenced by "payment_link" rows or "cr_report_line"
rows (off the top of my head) and probably others. When those
lines refer to these rows, either the referencing rows are
meaningless (likely this is the case for "invoice" rows). Or
they need to be replaced by a functional equivalent (this is
likely the case for "cr_report_line" rows). Links are supposed
to be on the "entry_id" field in the "acc_trans" table.