Sorry for the assumption! Specifically you should add this to your configuration.nix:
systemd.services.phpfpm-simanja.serviceConfig.ProtectHome = lib.mkForce false; # or pkgs.lib.mkForce if 'lib' is not in scope
When you declare services.phpfpm.pools.simanja NixOS will create a systemd unit named phpfpm-simanja.service which you can reference by systemd.services.phpfpm-simanja. By default the phpfpm module in NixOS generates systemd services which set ProtectHome = true;. ProtectHome = false will make your home directory inaccessible which is why you received the error mentioned above. ProtectHome = true; is a good default for production servers… though maybe not the best for development in your case.
I hope this explanation helps.