Now , it is solved in next trial again today. I found when I tried to claim my rel , the pop up instruction is as you see ,it want me to sign in metamask first. Prior to that , math wallet extension poped up and it told me my wallet is locked that is I noticed today. At that time , my metamask is logged in situation. That's why I removed math wallet extension and logged out from metamask ,and then tried to claim again ,this time pop up window made me log in metamask smoothly and achieved my claim. I think that is because of 2 reasons: (1) Math wallet may be a priori than metamask in my chrome browser.
(2) Actually rel acc wallet is already connected to metamask because it showed metamask balance and address. But rel acc wallet might not know itself connected to metamask due to some reason I don't know.
(FYI)