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sreenivasAppssreenivasApps 

Guest users are not permitted to perform this operation

I have a Process builder on case that invokes flow which updates a case with contact details. 
When case is created manually, it is working fine. When a case is created from site it is throwing above error.

i have given the following permissions
Contact - read
Account - read
Case - read, create
and Run flow permission in Profile for the guest user.
VinayVinay (Salesforce Developers) 
Can you try with edit permission on case?
RituSharmaRituSharma
Give edit access on case through profile. Also, create sharing rule to share(read & write) the needed cases with guest users.
sreenivasAppssreenivasApps
@Vinay and @RituSharma, thanks for the reply. 
There is no eidt Permission for the case for Guest Profile.
I tried with Permission set also, but while assigning the permission set it is showing the error that, "permission set has edit permission on Case, to assign permission set for this profile, need to remove edit access on case".
 
VinayVinay (Salesforce Developers) 
Can you check permission mentioned in below links which has details on guest users

https://help.salesforce.com/articleView?id=000334874&type=1&mode=1
https://help.salesforce.com/articleView?id=000313406&type=1&mode=1

Thanks