You are not setting the Marker ID's anywhere. You should also create a table with all the markers and loop trough it instead of creating every marker manually.
local markers = {
{692.7001953125,-3341.599609375,-3,"cylinder",1,5,250,45},
{664.09997558594,-3288.8999023438,4.8000001907349,"cylinder",0.5,170,240,10},
{683.90002441406,-3291,5,"cylinder",0.5,170,240,10},
{701.20001220703,-3260.5,4.0999999046326,"cylinder",0.5,170,240,10},
{712.59997558594,-3245.3999023438,3.7000000476837,"cylinder",0.5,170,240,10},
{745.20001220703,-3265.6999511719,4.8000001907349,"cylinder",0.5,170,240,10},
{759.59997558594,-3230.3000488281,7.0999999046326,"cylinder",0.5,170,240,10},
{816.59997558594,-3277.8000488281,9.1999998092651,"cylinder",0.5,170,240,10},
{841,-3328.8999023438,17.299999237061,"cylinder",0.5,170,240,10},
}
for markerid,_ in ipairs (markers) do
local m = createMarker(markers[markerid][1],markers[markerid][2],markers[markerid][3],markers[markerid][4],markers[markerid][5],markers[markerid][6],markers[markerid][7],markers[markerid][8])
setElementID(m,"m"..markerid)
setElementAlpha(m,0)
end
local function popUpMarker()
local marker = getElementByID("m"..math.random(1,#markers))
setElementAlpha(marker,255)
end
setTimer(popUpMarker,2000,0)
addCommandHandler("tp",function(player,_)
setElementPosition(player,660.90002441406,-3290.3999023438,5)
end)