Minecraft command syntax not working

You are missing the coordinates for /summon:

/execute @e[type=player,name=nimbusirrus] ~1 ~ ~ summon minecraft:husk ~ ~ ~ {CustomName:"Milo-the-Dragonslayer",CustomNameVisible:1,CanPickUpLoot:0b,Health:40,Attributes:[{Name:"generic.followRange",Base:25},{Name:"generic.knockbackResistance",Base:0.25f},{Name:"generic.movementSpeed",Base:1.1f},{Name:"generic.attackDamage",Base:10}],HandItems:[{id:"minecraft:diamond_sword",tag:{ench:[{id:16,lvl:5}]},Count:1},{id:"minecraft:diamond_axe",tag:{ench:[{id:16,lvl:5}]},Count:1}],ArmorItems:[{id:"minecraft:diamond_boots",Count:1},{id:"minecraft:diamond_leggings",Count:1},{id:"minecraft:diamond_chestplate",Count:1},{id:"minecraft:diamond_helmet",Count:1}],ActiveEffects:[{Id:12,Amplifier:0,Duration:2147483647}]}