I installed Plone 5.2.1 with buildout tool.
Now adding webdav-address to buildout.cfg as below, then re-run bin/buildout -c develop.cfg,
started the instant with bin/plonectl fg, but i cannot find the port 9800 for listening (means the WebDAV failed to be enable).
WebDAV support was temporary lost with Zope (it was removed from Zope and put into ZServer). Recent Zope versions (I do not know whether the Zope in Plone 5.2.1 is sufficiently recent) have again WebDAV support - but this works only with WSGI, not ZServer.
With Plone, the situation is a bit difficult. You can use it with either ZServer or waittress (i.e. WSGI). Configuration must go at different places. Ideally, the buildout recipe does the right thing but it may not yet do that. Check .../parts/<instance>/etc/{wsgi.ini|zope.conf}. For WSGI, the webdav definition must be in wsgi.ini; for ZServer in zope.conf.
Long story short: you need to provide your own wsgi.ini file containing the necessary configuration for WebDAV and provide to buildout through the wsgi-ini-template option (see zope2instance recipe docs).