i m using 3.8 version so what to do now?

It's been a long time since I used 3.x

Does it not have it's own sitemap generator?

vBSEO has been dormant for years and undoubtedly has some major vulnerabilities at this point. You are better off submitting a sitemap to Google Search Console yourself. Once Google starts crawling your forum, it doesn't really need frequent sitemaps anyway. Googlebot will find and follow your internal links to find all the pages it wants to index.

can you help me to make my seo . becos my forum post not showing in goole faster if creat one post it will show after 3-4days

Google indexes pages at its own speed.

If it is already indexing your forum, submitting more sitemaps won't speed up the progress.

How quickly a forum gets indexed depends on how frequently people are posting and how important Google thinks the content of your pages (posts, threads) are to its search queries.