Getting an error while spawning a mob in minecraft

Solution 1:

Change display:{Lore:["Gotta go faster-er"]}}}]}}}, to display:{Lore:["Gotta go faster-er"]}}}]}}}],

It's hard to tell, but I added a closing square bracket at the end there. Full corrected command is therefore /give @p spawn_egg 1 0 {EntityTag:{id:"minecraft:wither_skeleton",CustomName:"Eternal Archer",CustomNameVisible:1,Health:500,Attributes:[{Name:"generic.followRange",Base:500},{Name:"generic.knockbackResistance",Base:1f},{Name:"generic.movementSpeed",Base:0.5f},{Name:"generic.attackDamage",Base:15}],HandItems:[{id:"minecraft:bow",tag:{display:{Name:"Eternal Bow",Lore:["With his Eternal Bow, he could see his prey..."]},ench:[{id:48,lvl:10},{id:50,lvl:3},{id:51,lvl:1}]},Count:1},{id:"minecraft:rabbit_foot",tag:{display:{Name:"relic thing",Lore:["yeah"]}},Count:1}],HandDropChances:[0.01F,0F],ArmorItems:[{tag:{ench:[{id:0,lvl:7}]},id:"minecraft:diamond_boots",Count:1},{tag:{ench:[{id:0,lvl:7}]},id:"minecraft:diamond_leggings",Count:1},{tag:{ench:[{id:0,lvl:7}]},id:"minecraft:diamond_chestplate",Count:1},{id:"minecraft:blue_shulker_box",Count:1,tag:{display:{Lore:["What's in the Box?"]},BlockEntityTag:{Items:[{Slot:14,id:"minecraft:nether_star",Count:1b,tag:{AttributeModifiers:[{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:.5,Operation:1,UUIDLeast:902301,UUIDMost:657190,Slot:"offhand"}],display:{Lore:["Gotta go faster-er"]}}}]}}}],ArmorDropChances:[0F,0F,0F,2F],ActiveEffects:[{Id:10,Amplifier:3,Duration:2147483647},{Id:11,Amplifier:1,Duration:2147483647},{Id:12,Amplifier:0,Duration:2147483647},{Id:13,Amplifier:0,Duration:2147483647},{Id:26,Amplifier:4,Duration:2147483647}]}}

Note that I have not tested to see if there are further syntax errors.