Page life expectancy error in sql server
cntr_value = 788645


Your PLE is 788645, that is perfect and even more: it reflects that your server happily doing nothing.

So, don't worry and don't look your monitoring tool: it reflects something known only to its author

P.S. 788645 seconds = 9,... days, that is right, yor image also shows 2017-09-03 that, I think, is server last restart time. Since that day no more than 2Gb of data was passing through your buffer pool and it's still there, so, one more time: no memory pressure, no working activity on this server.