@ethicalhacker58
Unavailable chapters are unavailable for a reason. Usually it's due to removal by the uploader/scanlator. And since the uploading of work by active scanlators without their permission isn't allowed, then no, you can't add the chapter yourself.
As the option says, it's only for tracking purposes, i.e. to show that there once was a chapter there and it could potentially be reuploaded in the future if the scanlator ceases activity for 6 months (as per rule 2.4).
First-time uploads require manual reviews, so those also show up as unavailable before they're green-lighted.