Possibly a dumb question but are you using the database as the schema?
That is, in the UPDATE command it should be:
UPDATE <database>.<schema>.<table>
SET dest.[project_id] = stage.[project_id]
,dest.[cost_center_id] = stage.[cost_center_id]
,dest.[actuals_id] = stage.[actuals_id]
,dest.[quantity] = stage.[quantity]
*****\ lots of other columns \*****
FROM <database>.<schema>.<table/view>
JOIN <database>.<schema>.<table/view>
Looking at your code, my guess is that you are missing the schema. I think your code should be (presuming you are using the dbo schema):
UPDATE dest --This one is OK as you are using the alias
SET dest.[project_id] = stage.[project_id]
,dest.[cost_center_id] = stage.[cost_center_id]
,dest.[actuals_id] = stage.[actuals_id]
,dest.[quantity] = stage.[quantity]
*****\ lots of other columns \*****
From [ES_RTR].[dbo].[fact_Actuals_Order] dest
Join [ES_RTR_Staging].[dbo].[SU_factActualsOrder] stage
ONstage.actuals_id = dest.actuals_id
GO
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.