Try like restarting your browser or clearing your history or something. I was experiencing issues with IFC a while ago and it was because my browser was full
This is something Discourse should fix on its own end. They are continuously trying to improve the mobile version of their software. Best thing you can do is open a support request on the Meta community. If you include details like the device and browser you used it might help them to reproduce the issue. :)
Hey John. There is an easy temporary fix, just go to the categories section, and click ‘desktop mode’ and you can then invite. Once inviting is done, you can simply just click ‘Mobile Mode’.
But, in the long run, I myself don’t really know why this doesn’t work on phone.