The wikidata pages property (P304) is meant to be validated by a regular expression but there is a problem with this currently and I'm not sure whether the solution needs to be at the Wikidata end or the OpenRefine end.
This discussion resulted in some changes being made to the regular expression in Wikidata, but OpenRefine is still not able to use the regular expression successfully - which results in the error you are seeing (the issue isn't really an incomplete schema, but failure to validate the content of the schema I think).
I'm not sure if ultimately the problem is with the modified regular expression in Wikidata, or if that is now correct and the problem lies with OpenRefine - I might need to call on someone with some more expertise - I'm hoping Antonin, who wrote much of the Wikidata integration, might see this and comment. In the meantime the only thing I can think to suggest is to use the schema as you currently have it, but try using the "Export as Quickstatements" instead to generate a quickstatements file that can be used to update wikidata via the quick statements tool - this is a bit of a work around but looks to me like it would work.
Anyone looking to diagnose the problem, the error I'm seeing in OpenRefine log is:
21:12:46.756 [ command] Exception caught (381ms)
java.util.regex.PatternSyntaxException: Unknown inline modifier near index 2
(?'r'(?'p'(?'d'\d+)(?'w'[A-Za-z]+)?|\g'w'(?:\g'n'\g'w'?)?)(?:[-–]\g'p')?)(?:,\s?\g'r')*
^
at java.base/java.util.regex.Pattern.error(Pattern.java:2015)
at java.base/java.util.regex.Pattern.group0(Pattern.java:3034)
at java.base/java.util.regex.Pattern.sequence(Pattern.java:2111)
at java.base/java.util.regex.Pattern.expr(Pattern.java:2056)
at java.base/java.util.regex.Pattern.compile(Pattern.java:1778)
at java.base/java.util.regex.Pattern.<init>(Pattern.java:1427)
at java.base/java.util.regex.Pattern.compile(Pattern.java:1068)
at org.openrefine.wikidata.qa.scrutinizers.FormatScrutinizer.getPattern(FormatScrutinizer.java:68)
at org.openrefine.wikidata.qa.scrutinizers.FormatScrutinizer.scrutinize(FormatScrutinizer.java:80)
at org.openrefine.wikidata.qa.scrutinizers.SnakScrutinizer.scrutinize(SnakScrutinizer.java:57)
at org.openrefine.wikidata.qa.scrutinizers.StatementScrutinizer.scrutinize(StatementScrutinizer.java:36)
at org.openrefine.wikidata.qa.EditInspector.inspect(EditInspector.java:107)
at org.openrefine.wikidata.commands.PreviewWikibaseSchemaCommand.doPost(PreviewWikibaseSchemaCommand.java:92)
at com.google.refine.RefineServlet.service(RefineServlet.java:187)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserAgentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter.java:132)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.jetty.bio.SocketConnector$Connection.run(SocketConnector.java:228)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)