Don't try to manually create a sitemap. All your sitemap shows is the index page. It needs to have all the URLs you want indexed.
A phpBB discussion board is dynamic so you need to automate sitemap creation.
To create a sitemap, use the extension linked by warmweer.
That said, if Google is not crawling or indexing your website, this could be due to a number of things. A sitemap can help, but you need to check the basics first. Have you:
1. checked bots permissions in the ACP?
2. registered with Google Search Console?
3. within Google Search Console checked:
- whether Google is crawling your website?
- whether Google has indexed any URLs?
- used the Google robots.txt tester to test whether Google is blocked from crawling?
- used the Google live inspection test to see whether Google has any issues crawling specific URLs that are not the index?
Once you've ensured all of the above is OK, then look into the sitemap.
The first thing I noticed visiting your board is that everything redirects to the registration page after a few seconds. You shouldn't be trying to index pages that guests cannot see, certainly not a forum that appears to be by paid subscription only. A bot will probably not be able to crawl pages if they are automatically redirecting to the registration page.
Even if Google does index your search and returns it in the results page, users will be taken to a registration page, they will get frustrated and your bounce rate will be 100%, which will be terrible for SEO. Alternatively, they will use the Google Cache feature and view that so that they don't have to pay your subscription fees.
In the 5 seconds I could see the index before the redirection, the board looks very new. It will take a while for Google to discover and index your board, with or without a sitemap.