@Y_Guo - and anyone else having difficulty installing 21.1.2 on Linux:
On Linux the workaround is a little different. Try:
$ conda activate mfix-21.1.2
$ conda install jsoncpp=1.8.3
this got me a working MFiX on Linux - please let me know if this works for you.
We hope the installation goes more smoothly on the next release - sorry for the inconvenience.
– Charles