Can I release the remix?
If you do not have the rights to the original song, then without agreements with the copyright holders, you cannot release your remix. You need to sign agreements with the owner of the phonogram, with the author of the work, with the publisher. Sometimes all of these rights belong to one person or company.
Also, if you use someone else's sample or vocals in your song, you need to sign agreements.
Can I release the cover version?
Not unless you have a signed agreement with the copyright holder of the original song. To post a cover version on digital platforms, you need to obtain a mechanical license for the world. Often companies issue it only for publication in one country.