The medium tank in 1938 disappeared (because it was stacked under the 1943 medium tank ), making it impossible for players to develop medium tanks.
Solution：The file"common\technologies\NSB_armor.txt"in line 604 "position = {x=2 y= @1943}" of the file is changed to "position = {x=2 y=419}" (due to position of armor technology tree @1938 is not defined in countrytechtreview.gui file, only coordinate values can be used).