Unlike the Panasonic and Pioneer "global market" PAL/NTSC recorders, the Sony RDR-HXx80 and RDR-HXx90 weren't even "semi-officially" imported into North America. Sony was quite adamant they NOT be imported, so the grey market for them was nil. Your unit was either brought in by by someone who previously lived in another country, or was sold by one of several Hong Kong based recorder-hacker-dealers. For North America, Sony officially offered the sole Canada-spec NTSC-only RDR-HX780 variant, and that was it. So it is entirely possible timer recording is disabled in your multisystem model when the recording system is set to NTSC (all these global units were intended primarily as PAL recorders with PAL tuners and PAL epg systems: the NTSC compatibility was a "bonus" feature that wasn't always fully supported for all functions).
There are several options buried in the setup menus you can look at to try and solve this problem. The PAL/NTSC mode change involves more than one setting on some models. The "Input Line System" may already be set to NTSC, but the "Input Color System" may not allow timer recording unless you switch it from "Auto" to "3.58NTSC."
The Sony x80 and x90 recorders are based on a Sony/Pioneer hybrid design, so they incorporate some elements of the Pioneer interface. Some users of the Pioneer PAL/NTSC models have reported difficulty with the primary timer record system (the one that comes up when you press the Timer button on the remote). The primary timer system of the PAL/NTSC Pioneers (and many Sony variants) relies on an EPG signal embedded in PAL broadcasts: when used in North America, that signal is unavailable, and the timer refuses to engage. The workaround for the Pioneers is to enter the recorder main/home menu, click on the onscreen Timer button, and in the resulting submenu select the secondary "EZ Timer." This backup timer bypasses the PAL EPG, and permits manually setting the timer for NTSC line input (L1, L2, L3) much like a VCR.
I am not sure if the PAL/NTSC Sonys also have this backup timer for NTSC, but its worth a look thru the System and Setup menus on the off chance its an undocumented feature carried over from their Pioneer brethren. The Sony instruction manuals are totally useless on most points, made worse by the lack of diagrams showing the layouts of the setup menus.
One more workaround you can try is switching the unit to PAL mode, setting the timer, then reverting back to NTSC. This will only work if your TV supports PAL signals over HDMI, otherwise you won't be able to see the recorder menus after switching to PAL. You can check if your TV displays PAL over HDMI by downloading a PAL video (avi or DiVX), burning it to a DVD, and attempting to play it in your 890. If that works, a shortcut method of switching between PAL and NTSC mode is to hold down the front panel STOP button and then press the front panel INPUT button with your other hand. Each press of the INPUT button will cycle between PAL and NTSC mode. There is vague reference to NTSC mode details in pages 125-130 of the manual.