Tried to use a NULL entity!

[leeroy___]

I have literally no clue why this is happening.

local ply = FindMetaTable( "Player" ) -- line 1

local plydatafile = "cool/data_" .. ply:SteamID64() .. ".txt" -- line 18
I literally defined "ply" a line above.

Error:
[ERROR] addons/datastoringtest/lua/autorun/sv_testdatabase.lua:18: Tried to use a NULL entity!
  1. SteamID64 - [C]:-1
   2. unknown - addons/datastoringtest/lua/autorun/at_time_db.lua:18

[LoweChaser]

You can't get player's data from metatable, it's just a set of methods. Try to read your player data file in GM/PlayerInitialSpawn