The info you gave is somewhat limited but to start, the message you are getting is quite clear. You do not have json_module.mod file in your project tree. If you do have it elsewhere (from another project), you should add the directore it resides in, using -Idir option
Sorry if the question is unclear. The directory tree I wrote is just an analog to the actual directories. So, modules.f90 is actually a bunch of modules, and one of them is json_module.
Nevertheless, I think I need to tell fortls where to look for the modules so that it can find the definition. But, I don’t know how to do this.