Alarm is set to "User Clearable" but the alarm can still not be manually cleared in Spectrum
If the alarm was asserted before it was set to be user clearable then a user will not be able to clear it manually. Only alarms created after it was set to be user clearable will be user clearable.
This is functioning as designed.
You can use the Command Line Interface "create event" command to create the event that would normally clear the alarm.
Please reference the "Event Configuration" and "Command Line Interface" sections of the documentation for more information.