This means your slave account lacks enough free margin to execute the trade for the given lot size.
You need to adjust your risk settings in the trade copier settings for a smaller lot size or increase your margin/ capital in the slave account. To adjust your risk settings go to Configurator > Trade copier > Copier Configurator > Risk Settings > Adjust your risk settings to open smaller lot sizes. If this does not work you have to increase your margin / Capital in the account.