Unable to add a physical volume to volume group

I am trying to increase the space allocated for /dev/sda2 which is mounted as '/'. I have just added a new LVM Partition /dev/sda4 for 20GBsize and I need to extend the /dev/sda2 have that additional 20GB. The issue I seeing is there are no Volume Group and anything to create using vgcreate newvg1 /dev/sda2 /dev/sda4 ending in error:

[root@localhost cmanbin]# vgcreate vg1 /dev/sda2 /dev/sda4
Can't open /dev/sda2 exclusively. Mounted filesystem?
Unable to add physical volume '/dev/sda2' to volume group 'vg1'.

I have RHEL 6.8, installed as a VMWare machine on Mac system.