This normally happens on iOS 11.
It's possible that the AppStore was already open and is not showing an updated list of apps with available updates for download.
Here's what you need to do.
Close and re-launch App Store on your device
Tap the Updates tab at the bottom right
Swipe down with your finger, then let go. This will refresh the Updates tab.
You should now see the latest version of SeriousMD available for download.