with t as(
select '[{"Key":"ErrorCode","Value":"16734"},
         {"Key":"DocumentNationality","Value":null},
         {"Key":"DocumentAddress","Value":null}]' s
)
select json_value(b.value, '$.Key') Key1,
       json_value(b.value, '$.Value') value
  from t
  cross apply
  openjson(s) b

with t as(
select 1 customerId, '[{"Key":"ErrorCode","Value":"16731"},
         {"Key":"DocumentNationality","Value":null},
         {"Key":"DocumentAddress","Value":null}]' s
union all
select 2 customerId, '[{"Key":"ErrorCode","Value":"16732"},
         {"Key":"DocumentNationality","Value":null},
         {"Key":"DocumentAddress","Value":null}]' s
union all
select 3 customerId, '[{"Key":"ErrorCode","Value":"16733"},
         {"Key":"DocumentNationality","Value":null},
         {"Key":"DocumentAddress","Value":null}]' s
),
js as(
select customerId,
       json_value(b.value, '$.Key') Key1,
       0+json_value(b.value, '$.Value') value
  from t
  cross apply
  openjson(s) b
)
select *
  from js
 where key1 = 'ErrorCode'