Cannot write to _system tags that are read/write

I am unable to write to the <Channel Name>.<Device Name>._System._Enabled bit even though the help file says it's read/write.

TOP Server version 5.7 or earlier...

Right-Click on the Admin Icon in the system tray and select Settings. In the settings dialogue, select the Runtime Options tab. Check the box to "Allow clients to write to system level tags".

TOP Server version 5.8 and later...

Right-click on the Admin Icon in the system tray and select Settings. Select the User Manager tab. Double-click the "Data Client" user to open its properties dialogue. Check the box next to "Allow writes to system tags that...". You may also select or de-select individual items under that heading.

Earlier versions...

For TOP Server versions earlier than 5.x, this setting can be found under Tools ⇒ Options ⇒ OPC Settings - check "Allow clients to write to system level tags".