What can I do when a manuscript is Completed Reject after 5 months of the submission
You can certainly submit to another journal. You don't even need to make any changes to address the reviewers' comments, though it would certainly be advisable (otherwise you're likely to get similar reviews again).
Whether or not you can re-submit a revised paper to the same journal will depend on the journal's policy. The journals I know tend to treat outright rejection (as opposed to 'revise &amp; resubmit') to mean no resubmission will be accepted, but this may not be universal. Of course, there is a grey area: at what point does something become a new paper rather than a revised version of an old paper?