Is it required to have license to sing an original song and to post in Social media?

Covers are seen as your creation and not as the original artist's creation. It is so because you have added your own embellishments to the original song. Here are ways you could make your song/cover sound a bit different. If you are only going to sing over the original backing track, the musicians might still call it a copyright infringement. So alter the backing track by changing its tempo, changing the key or playing it on a different instrument. Or get hold of "karaoke" versions of the song from YouTube and credit the source in description. Usually karaoke versions are free to use. If you do not want all these headaches then just use karaoke apps. They have all rights of recently released songs. There you can freely express yourself in the way you like and share a link. There are many such apps in playstore and other store apps. But if doing just vocal cover wasn't your thing then go ahead and record all instruments. But always mention the original work in description with a notion of "Fair use".

Understand first that when you are planning to use any kind of existing thing for commercial gain then obviously original owners will claim and can sue you in court. Now for your problem , if you upload song for commercial gain then it can be blocked if owners wanted. Since now many fusion and remix are coming in market they have to pay some contributions to owners of song presently mostly are music companies.
For small scale publishing , does not matter more. So yes you can upload for your skills but do not market your posting for money gain.