I would like to find how to solve the issue of accurate closing price of any bar using a Uni-Renko Chart in Ninjatrader. Can this issue be solved using the Strategy Builder or it requires a specific coding.

Unirenko or Renko ... fill in bars where the price may never have been so....Strategy Builder will not solve your problem....the type of bar is what you need to determine where the price is.... one such bar is SiBackTest Bars from Shark Indicators here are some examples....Where the price is/was and where a unirenko type bar shows..... as you can see in some cases the price skips over bars....and was never there where the unirenko or renko type bar says it was....

In Renko and Unirenko you only can use High or the Low (up move /down move) in programing bots.
The open well is possible becuase you know the value for the size of the bar.