Due to the way the vanilla End dimension is coded, Recurrent Complex by default is unable to generate structures there. The workaround to this is to use the mod Just Enough Dimensions to replace the End dimension's world provider.

To do this, install and run JED so that its config folder generates. Then go inside its folder and create a plain text file with the name dimensions.json. Then post the following script in it:

{
    "dimensions": [
        {
            "dim": 1,
            "override": true,
            "dimensiontype": {
                "id": 1,
                "name": "Custom End",
                "keeploaded": false,
                "worldprovider": "WorldProviderEndJED",
                "require_exact_match": true
            },
            "jed": {
                "SkyRenderType": 2
            }
        }
    ]
}

Then go to the JED main config file and set the following values to "true": enableReplacingRegisteredDimensions, enableOverrideWorldProvider, enableOverrideBiomeProvider. You should now be able to generate RC structures in the End.