Appian CMIS connector to Alfresco -scsExternalSystemKey setting

Following https://docs.appian.com/suite/help/17.4/fnc_connector_cmi_a_cmigetrepoinfo.html

In expression rule,

=with(
local!cmisResult: a!cmiGetRepoInfo(
scsExternalSystemKey:"alfresco",
usePerUserCredentials: true,
atomPubUrl: "dxxx/.../atom",
repositoryId: "d0c720ee-6349-4334-8763-8f0cc73dx"
),
if(local!cmisResult.success,
local!cmisResult.result,
local!cmisResult.error
)
)

I have defined the third party credential as "alfresco", but still got this message:

Dictionary title: "Credential key not defined" message: "You do not have credentials configured for this system [key=alfresco]. Please navigate to the Third-Party Credentials page in your Settings and provide credentials for this system." detail: "You do not have credentials configured for this system [key=alfresco]. Please navigate to the Third-Party Credentials page in your Settings and provide credentials for this system."

I verified my user name, password, and atomPub with cmis workbench, which can connect alfresco. What is wrong with my setting to get the error: You do not have credentials configured for this system [key=alfresco]

Thanks in advanced.