Make sure the "Bypass Color Grades and Fusion Effects" button above the viewer in the Edit tab is not engaged (it is to the immediate left of the "..." button in the upper-right corner of the viewer).
If it is not engaged:
What is connected to the MediaOut node?
It needs to be connected to the result of your composition.
If you still have the original MediaIn node connected directly to the MediaOut node, you are basically telling Fusion that what comes in should be sent out (without being processed).
Try something simple as a test: place the playhead in the Edit page over some other clip (a victim for temporary testing) and switch to the Fusion tab.
Drag a Blur node (water drop) from the toolbar onto the line between MediaIn and MediaOut nodes until the line changes to two colors, then release.
This should result in the MediaIn node sending its output to the Blur node, which in turn sends its output to the MediaOut node:
- Screen Shot 2021-05-17 at 6.59.51 AM.png (19.51 KiB) Viewed 1178 times
With the Blur node selected, crank the Blur Size until the preview in the viewer is quite visibly blurred.
Switch back to the Edit tab and within a second or two it should render and show the result of that blurriness in the clip that you had selected.
NOTE: you can get rid of this temporary Fusion comp later by right-clicking on the clip and selecting "Reset Fusion Composition" in the context menu.