If you're unable to sign in to your Apple ID on your iPhone, first ensure that your device has a stable internet connection. Then, double-check your Apple ID and password for any errors. If you've forgotten your password, visit iforgot.apple.com to reset it. Make sure to check for any system updates, as outdated software can cause issues. If the problem persists, you can try signing out of iCloud and signing back in. Additionally, verify that your account hasn't been locked for security reasons. If necessary, contact Apple Support for further assistance.