I tried to use the dissolve function but that is the output (picture). The input for a map of the world with national borders, and my intention was to create to create the same map of the world without country's boundaries. Thank you.
When you use Dissolve or a few such Geoprocessing tools on huge datasets, they undergo tiled processing, resulting in such output.
So either you Dissolve the above layer (with tile boundaries) again.
or Merge the features in the layer by selecting all the features.
Thank you for the suggestion. I attempted to dissolve again, but it was taking too much time so I gave up. I thought I might reduce the quality of the map while keeping the original state's boundaries; what is your opinion?
If the outline of the polygons are bothering you, change the symbology of the patch to have an outline thickness of 0 (zero) in other words, no outline, just a fill color. It might look ok