Sergio oliva and Serge nubret (both dead) never made the cover of Flex magazine
Both Oliva and Nubret were at their peaks way before Flex started publishing.
The first issue of Flex was April 1983. Flex was a Weider mag and therefore IFBB only. Nubret was persona non grata with the Weiders back in 1976, having entered NABBA and WBBG contests, and later forming WABBA. The last Weider mag cover he was on was June 1972
Oliva had a more complicated relationship. He was disqualified from the 1973 Mr International for entering Dan Lurie's Mr Galaxy a few months before. But being the second Mr Olympia, Weider couldn't completely ignore him. Oliva was on the cover of Muscle Builder in May of 1978. After Flex began, Oliva was in the 1984 and 1985 Olympia, but coming in 8th was not going to get you on the cover.