Which of the following could the national government not do under the Articles of Confederation? a. Impose taxes
b. Declare war
c. Coin money
d. Establish a postal system

The duties of the national government are listed in the Articles of Confederation. What the national government cannot do under the Articles of Confederation  is to Impose taxes

There are some weaknesses in the Articles of Confederation became apparent. Some of them was that the Congress commanded little respect and had no support from state governments since its anxious to hold their power.

Others were Congress cannot raise funds, regulate trade, or conduct foreign policy without the use of the voluntary agreement of the states. To raise money and troops, Congress needs the permission of the states.

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this is a good question! i'll do some research and come back when i have the answer!