Transport ships require 1 population, like this.

In this case the population is a legion.

Your ship does not have any population.

If a ship that requires population doesn't have any, it can't launch until it gets some.  There is no information telling you why.  Your ship can't get population from the starbase, so it will never launch.

The only solution is to load an earlier save and don't send the ship to a starbase.  You can send it to core world with more than 1 population, then launch it from there.  You will be required to add population to it.  After that the ship can launch from a starbase because it already has the required population.

It the save is too far in the past, just forget about the ship.  It is useless.