You may still use a weapon you aren't Proficient in the use of, but you may not add your Proficiency Bonus to your Attack Rolls.

If you do not meet a Prerequisite for use of a weapon you may still use it, but all attacks using it will have Disadvantage.

Weapons and Stands:

Normal weapons cannot damage Stands. However, if you take the Energy Imbuement Feat, when a weapon is imbued (or incorporated into the Stand itself) it deals Stand damage instead and may now damage Stands

If your Stand wields a weapon or has a weapon incorporated into it, it may attack using its own stats rather than that of the User. For example, Strength becomes Power, Dexterity becomes Precision, and Constitution becomes Durability.