Not doing damage to Arachane

[AjTanim]

I have not played skyblock in a while and i reset my profile to start again, im using a spider sword and trying to kill arachane, in chat it keeps saying you can not do damage to this thing, some words like that, before this happened i used to be able to kill it and get its drops, but after i got the spider tamer quest to kill the arachane, i could not do damage, can anyone help or explain please?

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