If you are using a WeMos D1 Mini you can flash the pre-built PlayaWiFi.ino.bin binary to any devices prefixed with /dev/tty.usbserial- by running make autoflash and plugging in the WeMos.
Otherwise, open PlayaWiFi.ino in the Arduino IDE, select your board settings, then select Sketch > Export compiled Binary. Then run ./flash /dev/tty.YOUR_DEVICE_PORT ./PlayaWiFi.ino.YOUR_DEVICE_TYPE.bin (using Arduino IDE directly will not reset the flash storage the sketch uses). If you modify index.html, run make index.h before compiling.
You can also try updating the Makefile with your board info, then running make build.