I have created a flow where, i am updating a record using D365FO connector - Action : 'Update a record"
Where item number is mandatory setup in the action,
unique fields - Production order number
Field item number - Allow edit : No
When i update the record , am getting error "An error has occured.Update not allowed for field "Item number"
Please help me to resolve the issue
Please have a look at this post which is related to what you described.
I have tried to answer some of the similar issue in a generic answer to post: cannot update record in d365 finance - Power Platform Community (microsoft.com), I hope you can get some value from that.