I ran into the same problem today. Here is how I fixed it:
First ssh into pineapple using ssh firstname.lastname@example.org
Then you have to install the full version of python on the pineapple by using the command: 'opkg install python' (By default python-light is installed)
Now you can install pip: 'opkg install python-pip'
finally you install tinycss with 'pip install tinycss'
The whole process could take a while but it worked for me! This is a useful article for a beginner to the gui: https://blog.inspired-sec.com/archive/2017/01/10/cloning-captive-portals.html
Hope this was helpful!