My homebrew item was rejected because it says I didn't add the spell modifier needed. However, whenever I go to add the Armor of Agathys spell, it is not a valid option in the Spell section. Can this be fixed?

Here is a link to it (hopefully moderators can use it): https://www.dndbeyond.com/magic-items/1207175-armor-of-frost

Armor of Agathys is not a free spell so you would need to purchase it to add to homebrew.

Also you cannot publish anything with non-free spells/etc linked to it. 

You can use homebrew for personal use without issue. But there are restrictions for publishing them for the public to use. 

Discussion of a moderator decision or to appeal your rejected homebrew should be discussed with a moderator directly and not done in the forums. You can view the Site Rules and a list of moderators to contact here: https://www.dndbeyond.com/forums/d-d-beyond-general/news-announcements/2-site-rules-guidelines

My D&D group has an account with all the purchased materials that we share through campaign sharing, and we have the PHB, which should give me access to Armor of Agathys. 

And I understand about the publishing thing, that's fine.

Quote from corisande >>

My homebrew item was rejected because it says I didn't add the spell modifier needed. However, whenever I go to add the Armor of Agathys spell, it is not a valid option in the Spell section. Can this be fixed?

Here is a link to it (hopefully moderators can use it): https://www.dndbeyond.com/magic-items/1207175-armor-of-frost

 
Check your item again; It should be available once more with the associated modifiers entered.  Let me know what you think.

Quote from Sedge >>

Quote from corisande >>

My homebrew item was rejected because it says I didn't add the spell modifier needed. However, whenever I go to add the Armor of Agathys spell, it is not a valid option in the Spell section. Can this be fixed?

Here is a link to it (hopefully moderators can use it): https://www.dndbeyond.com/magic-items/1207175-armor-of-frost

 
Check your item again; It should be available once more with the associated modifiers entered.  Let me know what you think.

 Thank you! It looks great.