Most likely you're looking at a Generation 1 game. Shiny Pokemon didn't exist in Red, Blue, and Yellow, so shiny sprites aren't shown in a Gen 1 context.
Step 1
Shinies started in Generation 2 (Gold and Silver). Anything viewed in a Gen 1 context won't have shiny options. That applies to detail page sprites and tracker shiny toggles.
Step 2
View the same Pokemon in a Gen 2 or later game and shiny sprites are available. Pikachu has no shiny under Pokemon Red, but does under Pokemon Gold.
Step 3
In the Tracker, shiny tracking for Gen 1 games isn't available either. Same logic. You can track shinies for the same Pokemon in any later game.
Step 4
If shinies are missing for a Pokemon in Gen 2 or later, it might be a data availability issue. Some newly added Pokemon or rare forms may not have all sprites uploaded yet.
Step 5
Make sure you have the right game selected. Switch between games using the Generations tab or game list on the detail page.