Field is not Writeable

[Max_g]

I am using Dataloader to UPSERT records into a custom object that has a master detail relationship to Accounts. 

Trigger will not allow me to set the account id on my detail object for the Account Id field. 

If I take the assignment out of the trigger then my error is required field is missing.  I don't have the account Id in my data.

What am I missing in my setup?

[Max_g · Accepted solution]

Got the issue resolved.