"Why don't you want to make supplier and machine column accept null?"

Because I think it would be incorrect.

One of the transaction is that Supplier gives fuel to operator  so supplier...

You got everything correctly over here

"How do you intend to track "transactions"?  I seem to be that the operator gets a "batch" of fuel and then the operator give that...

Yes, I was thinking about that too. But I don't know what should be in the 3rd main table.

Should it be

MainTable(id, DieselReceived_ID, DieselTaken_ID)  ?

I intend to learn database design. I watched 1NF, 2NF, 3NF and BCNF on youtube. But still I am not able to understand it properly and cannot use them correctly....

Like this?

DieselReceived(id, supplier, operator, taken_time)

DieselTaken(id, operator, machine, given_time)

Is this okay?