If you are trying to create a SSAS Tabular model and if you are getting error similar to following one then you are reading appropriate post as I am going to explain the solution for resolving the issue.
Error looks similar to following one;
OLE DB or ODBC error: Login failed for user 'Domain\instancename$'.; 28000.
A connection could not be made to the data source with the DataSourceID of 'e2d72aea-e51d-4816-a6f6-c47e98716eff', Name of 'SqlServer localhost AdventureWorks2012DW 2'.
An error occurred while processing the partition 'DimCustomer_80fa8624-8c3f-43f0-9e17-950292667334' in table 'DimCustomer_80fa8624-8c3f-43f0-9e17-950292667334'.
The current operation was cancelled because another operation in the transaction failed.
If you are trying to create a tabular model with "Impersonation Information" as "Service Account" then you will have to add "NT AUTHORITY\NETWORK SERVICE" in the "Security" of relational database.
Connect to relational database and expand "Security" folder, add new login i.e.NT AUTHORITY