I have a video uploaded on youtube, it ranks #6 on their for the long tail keyword but when I search for the keyword on google, it has not been shown there yet.
How can I improve this for both search platform, google and youtube?
Take a look at your Title & Description. These 2 things play a big part in how your video will rank.
Also keep in mind that backlinks are just as important (if not more important) for a video then they are for a website. You want to get links to your videos from social media, other websites, etc.
Well, if you want to rank your video in SERP and YouTube as well then you have to make it as per the algorithms of Google then only it can be viewed on both SERP and YouTube that too at the top.