Trump did not tell McGahn to lie, nor to fire Mueller.
The very article you site even says it accurately. Trump told McGahn to go to Rod Rosenstein that Mueller had a conflict of interest, and had to go.
Mueller did, of course, have a glaring conflict of interest in that he was VERY close to James Comey, and in the case of Rosenstien, the prosecuter was a witness. This would not in any way end the investigation but would meet the requirements of the laws that are in place to prevent a biased or rigged investigation
Everyone on Mueller’s team was conflicted. They were all Trump haters, Hillary Clinton supporters, and a few were Hillary Clinton’s personal attorneys or attorneys for the Clinton foundation.
Pointing this out and attempting to appoint a new investigator is not tantamount to “telling McGhahn to fire Mueller.”