Can’t share reminders anymore

Probably you are sharing with someone who does not have an Apple ID.

In the Reminders app , use iCloud to share to-do lists.

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Collaborate on a list using iCloud

1. While viewing a list, tap .
2. To change the access and permissions, tap the share options below Collaboration. You can set either of the following: ... 
3. Choose how to send your invitation (for example, using Messages or Mail).

Share lists and collaborate in Reminders on iPhone