1) The timeframe you selected in the SMC HTF Settings is a lower timeframe than the timeframe you are viewing.
In the below image you can see the Higher Timeframe selected is the 1 Hour but the chart I am viewing is the 4 Hour. This will make the cells in the Trading Panel that read this timeframe blank.
2) The Swing or Internal structure may be too far back for the indicator to read it. In the image below, you can see that we are currently viewing the 5 minute chart, and wanting to use the Daily as our Higher Timeframe. The Daily Swing Structure is too far back in the past on the 5 minute timeframe to be able to read it.
The trading panel lets you view up to two different timeframes, in addition to the one you're currently using. So in total, you can see information from three different timeframes at once.