Ag command can't work normally!

[aixiaoxiaoyu]

ubuntu 14.04(64)
vim version 7.4.52

Hi！I have install silversearcher-ag and mileszs/ack.vim . When I use ": Ag！"，vim reports an error .As follows:

[linteck]

You need install "the_silver_searcher".

[aixiaoxiaoyu]

Hi! I have determined to install the the_silver_searcher,but the error still persists.