I created my custom package with immortality for a sim but faster needs gain & skill leveling and anti-needs decay are not working.

Assuming this is a question, I think you need to post in the create section to ask for help.

Are you sure you did it correctly? You have to add it to the core buffs section and if it isn't part of that section it won't work.

How do I add Skill & Statistic Multipliers, Skill Tag Multipliers, Need Decay Multipliers, Relationship Change Multipliers, Effective Skill Tag Modifiers, Effective Skill Modifiers, & Commodities to the Core Buff in Sims 4 Mod Constructor V3?

Where do I find core buffs section?

"Input the instance key of a resource from the game" or "Reference an Existing Element?"

Core buffs section is in the general section of the trait. Once you add that, you have to build the tree, then you go into that "buff" tree, from there you will find the general modifiers drop down, and that's where you'll need to reference the game resources to make skills and needs work properly. You have to make sure you are referencing them from the game resources specifically. If you use "reference from an existing element" then you are trying to add custom resources that you've created which won't work for skills and needs that come with the game. That would only work if you created a specific custom buff or trait you wanted referenced that you created.