Could you follow this article to troubleshoot this issue first?
Error: We encountered an error while trying to connect to . Details: "We reached the data gateway, but the gateway can't access the on-premises data source."
We were unable to connect to the specified data source. Be sure to validate the information provided for that data source.
Within Show details, you will see an error code of DM_GWPipeline_Gateway_DataSourceAccessError.
If the underlying error message is similar to the following, this means that the account you are using for the data source is not a server admin for that Analysis Services instance. Learn more
The 'CONTOSO\account' value of the 'EffectiveUserName' XML for Analysis property is not valid.
If the underlying error message is similar to the following, it could mean that the service account for Analysis Services may be missing the token-groups-global-and-universal (TGGAU) directory attribute.
The user name or password is incorrect.
Domains with Pre-Windows 2000 compatibility access will have the TGGAU attribute enabled. However, most newly created domains will not enable this attribute by default. You can read more about this here.
You can confirm this by doing the following.
Connect to the Analysis Services machine within SQL Server Management Studio. Within the Advanced connection properties, include EffectiveUserName for the user in question and see if this reproduces the error.
You can use the dsacls Active Directory tool to validate whether the attribute is listed. This is tool is normally found on a domain controller. You will need to know what the distinguished domain name is for the account and pass that to the tool.
dsacls "CN=John Doe,CN=UserAccounts,DC=contoso,DC=com"
You want to see something similar to the following in the results.
Allow BUILTIN\Windows Authorization Access Group
SPECIAL ACCESS for tokenGroupsGlobalAndUniversal
To correct this issue, you will need to enable TGGAU on account used for the Analysis Services windows service.
Another possibility for user name or password incorrect
This error could also be caused if the Analysis Services server is in a different domain than the users and there is not a two-way trust established.
You will need to work with your domain administrators to verify the trust relationship between domains.