Cant program direct tv remote with my new sylvania lc320slx

The remote is only programmed with only so many codes. Sadly, It doesn't hold all code for every TV made.
But you can try making the remote search every code stored inside to see if one of them will work.
Here are the instructions form the DIRECTV web site:

You may still find a setup code that works for your device by stepping through the library codes in your remote. The following instructions will help you do this.

StepNSet

• Set the "Mode Switch" to the desired mode such as TV for a Television.
• Press and hold the "Mute" button and the "Select" button until the visible LED blinks twice.
• Enter the code "991" followed by the "device number"
  1. for Televisions
  2. for "AUX 1" devices - VCRs or DVDs
  3. for "AUX 2" devices - Audio
• You can step through the setup codes for this mode by pressing the "Channel Up" key to go forward through the codes or pressing the "Channel Down" key to go back to the previous codes.
• You can try out each code by pressing one or more function keys such as power, volume or digit keys.
• When you've found the setup code that works for your device, press the "Select" key.