I can sign into my Starbucks app on my PC, but not on my iPhone.

Hi lin51mon - Welcome to Apple Support Communities. I'll be glad to assist

Now I love Starbucks and I'd be gutted if I couldn't log in to my app too so I'll be glad to help you -

1. Are you logging into the correct app? Starbucks have recently changed their app if you are in the UK and you must download the Starbucks UK app
2. If you're using the right Starbucks app, if I remember rightly this is just Starbucks in the US - then try uninstalling and reinstalling the app.
3. If it continues you might want to contact Starbucks for your own troubleshooting, but if I can be of any further assistance please don't hesitate to let me know