You should be able to hover over the cruiser button and it will say at the top of the info card what the reason is (probably not researched). Also, if you try to click it and then go in the research tree, there will be a button glowing blue telling you what research you need to build the ship.
You need to build enough military labs (the naming depends on faction) and then unlock the specific cruiser research that you want. When that is complete, you will be able to build that type of cruiser.
Hope this helps!
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