Dear Fezekile
This error is generated by GEE and not by SEPAL. It means GEE is not able to render the results of the processing. Some things you can try to reduce the processing load in GEE and render the image are:
1. Using re-tile option when rendering in sepal (using the properties icon [the cog on the upper right of screen] try selecting re-tile 4).
2. Using only 1 imagery source (e.g. only Landsat 9 or Landsat 8)
3. Restricting the time period to less than the entire year (to reduce the number of acquisitions being considered for the final composite)
What image source are you using? What time of year do you want to emphasize in the composite?
Best
Erik