Whever you start a Nether Reactor Core a message appears saying "All players need to be close to the reactor.", when I am the only one in the world.
Samsung Galaxy S4
MCPE-8193
Nether Reactor Bug
Resolved
Invalid
0
(Unassigned)
Unconfirmed
Phone - Android - Samsung Other (Specify in description)
Android 4.4.2
0.11.0 Beta 1
Retrieved
From your photos, it appears you are standing a block lower than the base of the reactor, in which case this Works As Intended.
Please attempt standing level with the reactor and comment your result here.