I am on an iPhone 8 with the iOS 13 update and I am no longer able to use spell slots; by either clicking cast or by clicking the spell slot itself. Ive experimented with a friends iPhone 7s and it still seems to be functioning normally. 
I’ve tried closing safari and restarting my phone. Ive also tried reloading the page and closing it. 

Just to clarify, you are using the web version and not the app? (Pretty sure character sheets havent been added to the app yet, but I don't use the app, so not sure).

Hi there Nithal,

I'm sorry to hear about the problems you're experiencing. I have a couple of steps you can try. Hopefully these will help get your sheets in working order on your device again.

1 ) Try to log-out and log back in.

2 ) Clear the cache on your phone. Here's a guide to do that for iPhones and iPads. 

Could you also please provide a link to the affected character? Please use the link from the address bar, not the share link.

Are you experiencing this issue using other characters?

I was only experiencing it on the one but closing the tab, logging back in, and navigating to the character seemed to set it right. Still not fully sure why it was only the one character of mine that was having the issue.