This error takes place when flow runs in the context of a Guest User – like Record is being created from public site or public webhook etc.
This happens because Guest User is not allowed to create chatter post i.e. Feed Post.
There are possible below approaches:
1) Using Platform Event : Guest user can publish platform events. So you just have to Write a trigger on platform Event that will insert a FeedItem i.e Chatter Post.
2) Using Apex Email Service: Guest user execution will send an email to an email alias and this email alias will be associated with an apex service, which will create chatter post. Here is more details how to use email apex service: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_email_inbound_what_is.htm
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