Spawn

[
{
"mob" :"mowziesmobs:lantern"
"biome" : "cavesandcliffs:lush_caves" ,
"result" : "allow"
}
]
Spawner
[
{
"mob": "mowziesmobs:lantern",
"weight": 5,
"groupcountmin": 1,
"groupcountmax": 2

}
]

Two errors:

'allow' is default. In addition to the allow rule you have above you also need a deny rule to deny the mob in all other biomes. i.e. don't specify a biome there.

I'm assuming you use 1.16.5? If so then use the correct syntax for spawner (see the wiki). You can't use potentialspawn keywords and syntax for spawner