It very probably is not a software but a hardware issue. Did you have a look into this thread and already tried a painting app? If you already did and the result shows that the left part is still not working, I’d suggest to contact support (https://fairphone.zendesk.com/hc/en-us/requests/new) but please be aware that they are still really, really busy and understaffed. So, if you don’t get an answer within a week try to call them. Else, you could also try to dis- and reassemble the screen. See here how to do this. Make sure to take out the battery first before disassembling the screen.