You cannot combine Weakness and Sharpness on the same weapon in Minecraft
Sharpness is an enchantment for weapons
Weakness is a status effect, not an enchantment
To apply Weakness to a target, use a Potion of Weakness or a Splash Potion of Weakness
You can also use a Lingering Potion of Weakness
In Java Edition, you can use the command `/effect give @p minecraft:weakness
In Bedrock Edition, you can use the command `/effect @p weakness
To get Sharpness on a weapon, use an enchantment table, anvil, or enchanted book
To have both effects involved in combat, use a Sharpness weapon against a target affected by Weakness
Weakness reduces melee damage dealt by the affected entity
Sharpness increases melee damage dealt by the weapon user