What Privileges Are Necessary to Approve Any Approval Transaction?

Summary:

We have a requirement where the HR user has been assigned a custom role to approve approval transactions but hasn't been assigned any abstract role (employee, line manager role). 

Currently, HR user is receiving approval notifications under the Bell icon, but when HR user try to respond to this approval, they get a blank screen.

Please advise what privileges we need to add to this custom role to allow the user to approve an approved transaction.

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