Hello there, Steve.
It sounds like you're trying to add a comment to a photo or video in a shared album. The following Knowledge Base article offer sup some great guidance on how to do just that:
iCloud: Comment on or like a photo or video in a shared album
You can comment on or like any photo or video in a shared album you own or subscribe to. If you own a shared photo album, you receive a notification when a subscriber adds a comment to or likes a photo or video in your shared album. If you are a subscriber to a shared album, you receive notifications when another subscriber has commented on or liked a photo or video only if you’ve already commented on it or liked it.
Note: Shared albums are called shared streams on iOS 7 devices.
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In the Photos app, tap Shared at the bottom of the screen.
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If you want to view a different album, tap Sharing at the top, then tap the album you want.
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Tap a photo or video, then do any of the following:
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Tap Like.
Tap again to stop liking the image.
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Tap “Add a comment,” enter a comment, then tap Send.
Thanks for reaching out to Apple Support Communities.
Kind Regards.
