Unfortunately, the specific action "Attach to running browser" has been deprecated in the Power Automate for Desktop (PAD) editor, and there is no direct way to create new instances of this action in the latest versions of PAD. Since you mentioned that you can copy it from an existing flow, your best option would be to find a colleague or a community member who has an older version of PAD (from 2024 or earlier) and can share a flow that contains this action with you.
Alternatively, you might consider using other actions that are currently supported in the newer versions of PAD to achieve similar functionality, depending on your workflow requirements.
If you need further assistance, you can also check the Power Automate community forums for additional support or workarounds from other users who may have faced similar issues.