How would you find the Merkle Root with a Block that contains 6 transactions? Let's use block #100205
https://www.blockchain.com/en/btc/block/0000000000033d0de6f4ebe93b3b142424cdbecf4bd98b948e856c4a55f09f76

This block contains 6 transactions. After the second level of hashing you end up with 3 nodes. What is the fix for this? Do you create two 5's and 6's?
You would duplicate the hash of the last two txs. The tree would "look" something like this:

             abcdefef
              /  \            
           /        \          
        /              \       
      abcd             efef     
      /  \            /  \    
   /        \      /        \  
  ab        cd    ef        ef 
 / \        / \   / \  
a   b      c  d  e   f