Trying to amend an earlier return ('18), I get two reference codes 5-005-466 and 3-451-977 and 1040X error that my "calculations could not be completed. Any ideas?

if broker supplies you a completed 8949 showing all your activity that is a  detail report.

TurboTax doesn't offer any way to e-File a detail report so if you want to use the 8949 supplied to you, you have to file on paper via US Mail.

For 1040X that's no problem since it has to be mailed anyway.

To add the 8949 use the summary option to enter  the subtotals on your detail 8949.

Then you will have two 8949s to include with form 1040-X, a summary 8949 and a detail 8949.

Keep a copy of the detail 8949 for your records.

In fact,  keep a copy of your submitted  amended return for your records.

So I can just ignore the error messages TurboTax is generating?  Do you think it will print fine despite them.

By the way, thanks "hugely" for your prompt answer to my question!  I 

really  appreciate it!

to amend a return you must print the revised return and examine it carefully until you are satisfied. TurboTax is not very helpful  with this.

When you are satisfied, 1040-X should reflect what you did.

You have to review the 1040-X yourself also.