you can try the follow ways:
1, Enable "Allow users to create site pages"
Go to SharePoint admin center -->Settings-->classic settings page-->
Ensure "Allow users to create site pages" is checked.
Restart your browser and re-login, then see if the problem is solved.
2, Add the content type to the library
In Site Pages Library,
go to "Library Settings"-->
In the Content Type Section, click on "Add from existing content types"-->
Search for "Site Page" from the list (you can simply select one item and press "S" on your keyboard to take you to all the content types starting from S) -->
Select "Site Page" and click Add and then click on OK.
Go back to Site Pages Library, click on New and you should be able to create Site Page now.