Signing in leaves the help center, goes through Atlassian, and comes back. Browsers guard that round trip closely, so most failures come from browser settings and have nothing to do with your account.
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Leave the private window. If the widget says Can't log in from a private window, you are probably in a private or incognito window. Open the help center in a normal window and try again.
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Allow cookies for the help center. If your browser blocks all cookies, it cannot keep your session.
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Turn off tracking blockers for this site. Privacy extensions and some company filters block the cookies and site storage that signing in needs.
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Check the account itself. Open the Jira Service Management portal directly and sign in with the same account. If that fails too, the problem lies with your account.