Hi, if you are getting this error only for previous period order, it is related to missing standard cost issue only. You can perform a small quantity of GR in current period (use the posting date from when the released cost is available) and recalculate the variances with the correct status of the order (DLV or Teco). That would fix your issue.
The other option is, you can also reverse all your GR and repost them in current period with the released standards.That way also you will not see any missing target costs errors that might be also resulting "cannot calculate variances" error.
If this error is wide open for all the order, then you need to check if the scrap variance is flagged for variance variant in OKVG.
Thank you,
Srinath