How to make a KHCO3 Buffer solution in RO water?

90 mg is very less. The moles are 0.9 mmol. But what I said add 124 mg potassium carbonate as well to get a real buffer.

Yes pH meter is calibrated daily with pH 7.01 and pH 4.01 buffer solution. Made the experiment multiple time to avoid errors. Got the same results. Mixed 90 mg from KHCO3 in 1 Liter of RO water.

Do potassium harm your plants? If not increase on 10 times means 900 mg KHCO₃ and 1240 mg K₂CO₃.

The Problem is you dont have a buffer. A buffer you would have with a mixture of potassium hydrogen carbonate and potassium carbonate, you add only one component. You add phosphoric acid as well Mixtures of hydrogen phosphate and dihydrogenphospate working as buffer as well.
But if I see your numbers, did you calibrate the pH meter. The first 3 days it's a slowly increased then the pH drops at 5th. day. Make no sense.
How much bicarbonate did you add.

I see, but how can I calculate the amount of KHCO3 that I should add to get a good buffer in my case?
Now it is 90 mg/L.

Probably you have to change to Di-Potassium hydrogenphosphate . Carbonate is to instable.

Understood.

What should be in this case the total amount of KHCO3 in 1 Liter of RO water which I should add? You are telling that 90 mg is too low.

Is this not a buffer cause of the solution can't hold the pH against pH raising? Like not able to keep pH against alkaline elements?

I can have an overall number of 1,2 mS/cm of conductivity. This is 840 ppm, which is 840 mg/L of dissolved solids.

So if I add 900 mg of KHCO3 there would be no place for other elements, like the nutrient solution. I know it is hard to calculate it as we don't know what elements would the plant uptake or what elements would the root system put inside the solution, but can we set a minimum amount to put inside from KHCO3 in order to keep the solution stable for at least 2 weeks? If I can achieve it by holding the solution in a jar without touching the plant it would be OK!!!

Part of the problem with a bicarbonate buffer system is that it is sensitive to the amount of CO₂ in the air, so ensuring a stable pH with a bicarbonate buffer requires regulation of the amount of CO2 in the air (e.g se https://en.wikipedia.org/wiki/Bicarbonate_buffer_system).

Also, while buffers can resist pH change, there is a buffering capacity related to the total concentration of buffer present.  Higher concentrations of the buffering agent will resist larger amounts of acid/base, but if acid/base is continually added (without adding additional buffer), then eventually the acid/base will win out.