I've shopped already this morning and fortunately had no issues scanning my Walmart receipt. It read the bar code right away. Sorry to hear you're experiencing issues however!
There were many who complained of not being able to submit their Walmart receipts Saturday evening and into Sunday morning. Even the Walmart app would not read the barcode.
On Sunday, I took back the three items I bought Saturday night and had them ring them up again. The new receipt worked fine.