There were three bumper covers for the 2012 Sentra, so you probably got the wrong one for your trim.
Base and S trims took Nissan P/N 62022-ZT51J and was available 2010-2012.
SL trim was P/N 62022-ZT50J and was available 2010-2012.
SR, FE+ and SR FE+ trims were P/N 62022-ET80J were available 2007-2012.
There were two grilles for the 2012 Sentra:
Base, S and SL trims took P/N 62070-ZT50A
SR, FE+ and SR FE+ trims took P/N 62070-ZE90B
So, it sounds like you got the wrong cover and either need to get a bumper cover for a 2010-2012 Base or S trim Sentra or a grille from a 2007-2012 SR, FE+ or an SE FE+ trim Sentra.
Base and S trims took Nissan P/N 62022-ZT51J and was available 2010-2012.
SL trim was P/N 62022-ZT50J and was available 2010-2012.
SR, FE+ and SR FE+ trims were P/N 62022-ET80J were available 2007-2012.
There were two grilles for the 2012 Sentra:
Base, S and SL trims took P/N 62070-ZT50A
SR, FE+ and SR FE+ trims took P/N 62070-ZE90B
So, it sounds like you got the wrong cover and either need to get a bumper cover for a 2010-2012 Base or S trim Sentra or a grille from a 2007-2012 SR, FE+ or an SE FE+ trim Sentra.