Trying to get mobs to spawn when the player passes through an area

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Okay assuming this is what you want going just from description then yeah here you go. Note I am not 100% sure this is multiplayer friendly but you can try

1.First either summon an armorstand with /summon armor_stand ~ ~ ~ {Tags:["Near"]} or go in to creative place down an armorstand and type in the command you see in the picture below what ever is easier for you. Note that the Tag name "Near" can be changed to what ever you want, but then you have to stick to it. If you're feeling confused just use "Near"

2. Add a scoreboard named what ever you want, I used Near /scoreboard objectives add Near dummy

3.Put down 4 commandblocks just like in the picture below make sure they are all set to "Always Active", the first one to be repeating, the rest as chained and the last one as chained with "conditional"

4.In the first commandblock paste in execute at @e[tag=Near] if entity @p[distance=..2,scores={Near=0}] run scoreboard players set @a Near 1

5. Second commandblock paste in execute at @e[tag=Near] unless entity @p[distance=..2] run scoreboard players set @a Near 0

6. Third commandblock paste in execute if score @a[limit=1] Near matches 1 run execute at @e[tag=Near] run summon zombie ~ ~ ~

7. Last one "scoreboard players set @a Near 2"

8. Now just write /data merge entity @e[tag=Near,limit=1] {Invisible:1b} in chat to Hide the armorstand

Okay super short explenation cause I've already spent 30 min on this. Command 1 If player is less than 2 blocks away from the armorstand set scoreboard to 1. If you want to change how far away change ..2 to ..x x=what ever distance you like .. means everything under x.

Command 2 if player is not near armorstand set score to 0

Command 3 if score = 1 summon mob this case zombie you can change ~ ~ ~ to cordinates just not that it is not relative to the armorstand

Command 4 if zombie has been summoned set score to 2 so that it won't spawn 1 billion zombies and allows for only 1 zombie to spawn