A question about my mace

Why i can't add multiple mods in my mace? I can add only "Can have multiple Crafted Mods" and "Attack Speed"

Thanks.

You have to craft the Can Craft Multiple Mods on it before you try to craft multiple mods. But you only have 6 mods available. If it already has 4 then it would be pointless because the multiple mods and the mod you add would take up the last 2 spots.

Remove attack speed then have 3 mods, after add "Can have multiple Crafted Mods" then have 4 mods, after add Attack Speed and have 5 mods and after i can't add other mod for complete the 6 mods, i can't understand why.

Did that mace drop as a rare? If so it has the minimum number of mods a rare can have (4).

This seems to likely be the case.
Your mace probably has:
Prefixes: (phys %) (hybrid phys% +acc) (+flat phys)
Suffixes: (+flat acc) - probably a low roll.

If you want to multimod this item, first craft Prefixes Cannot be Changed, Scour the item to remove the accuracy suffix and the crafted mod, then craft multi-mod, attack speed, and whatever you want the last suffix to be.

Your "three mods" are likely four affices. Don't invest too much time/currency into crafting this item until you understand what that means.