Hi Bocaj,
For system alias defined in SAP Gateway, do not need to input SID and client.
If you want to configure it for SAP Fiori, make sure all the prerequisite steps are done mentioned in SAP Service Marketplace - SAP Fiori Apps -> EE0_NWG20_BB_ConfigGuide_EN_XX.
Additionally, check if the component required in gateway is installed or not.
Regards,
Ning