Create a date column based on two numeric fields in DirectQuery mode.

Ah, sorry - I missed that you're using DirectQuery mode. 
In that case you could try to add the column via SQL through the advanced connection settings instead:

Then remove the Navigation-step below your Source-Step.

Hi telesforo1969, according to the error-message the table doesn't exist in the database with that name. Are you sure that your is in the dbo-Schema? 

You can confirm which schema your table belongs to if you connect normally via the SQl-connector and check the code in the "Navigation"-step (or by checking it directly on the SQL Server if you can access it)