That's not really true. A mistake (in physics or elsewhere) can be small or big. The mistake you made was fundamental, indicating you didn't really undertsand some basic physics.
It's a bit like confusing velocity and acceleration. It may appear to be a small mistake (e.g. units are m/s and m/s²) but the meanings are very different.
You used a formula giving force instead of the formula giving energy. Though the formulae look similar and are related, they mean very different things.
You never answered the question I asked in Post#8. Here is the question, slightly revised, if you would like to answer it:
You have 2 point charges, Q1 and Q2, separated by a distance d.
What is the formula for the force exerted on each charge?
What is the formula for the electrical potential energy stored?
Refer to your notes and/or textbook if required. And post your reply.
If we can't learn from our mistakes, we are doomed to repeat them!