There is a bug in the file OpenMMFortranModule.f90 for OpenMM 7.7.0
If you can find this file under your ~/anaconda3 directory, you can edit it.
All of the 3 instances of 'long long' need to be replaced with 'integer(8)' I think.
For me, the file I needed to change lives in
I found it by doing
find -L . -name OpenMMFortranModule.f90
in my home directory.
WARNING: conda may not be happy that you changed a file by hand. You may get messages when you update, install, or uninstall software in the future.