|"Then why, please tell me why did Mueller NOT charge him with obstruction?"|
Come on, Rich - your question makes it obvious that you haven't read the report, or at least the Executive Summaries. Have you read them at all?
The short answer is that he couldn't, due to DOJ policy.
On Page 1 of Volume 2, Mueller explicitly says that he declined to make the traditional "binary determination to initiate or decline a prosecution" due to the OLC's opinion that a sitting President cannot be indicted. He followed DOJ guidelines which said that, despite the evidence, he could not indict. On page 2 he says. "If we had confidence after a thorough investigation of the evidence that the President clearly did not commit obstruction of justice, we would so state."
What more do you need?
Mueller found evidence of obstruction, which is well documented in Volume 2, but he was constrained by DOJ policy to not indict. What he did was lay out the evidence for others to review and decide. Bill Barr took less than 48 hours to allegedly review a 400 page report and decide the evidence for obstructon was insufficient. Three weeks after he made that statement and the Trump team spiked the football and celebrated, we finally got to see the redacted report.
What it actually said did not agree with Barr's assessment.
Meanwhile more than 500 former Federal prosecuters have reviewed that same evidence and signed a letter saying that, in thier opinion, the evidence presented was sufficient to support one or more charges of obstruction. I trust them more than I trust Barr, who auditioned for the job by writing an unsolicited memo arguing that the President can not commit a crime. Any layman with no legal knowledge can read that report and realize that Trump was trying to stop the investigation.