You want to create a Wiki page under the Pages library of your Modern SharePoint Communication Site but you noticed that the Wiki Page option under the New tab is missing.
You can enable the option to create the Wiki Page content type. Navigate to Library settings for your Pages library by pressing the settings cogwheel on the SharePoint suite bar and selecting Library settings from the dropdown.
When the Library Settings pop-up appears, press the More library settings link.
In the Site Pages > Settings page, click Add from existing site content types link under the Content Types section.
Scroll down and select Wiki Page from the Available Site Content Types field. Press the Add button to move the selected item to the Content types to add field and hit OK.
Now go back to your Pages library and you should see the option to create Wiki page:
That's it! You can now select this option to create a Wiki Page any time.