Taken from SAP NOTE: 1648302
Solution
The following steps to be done manually as POWL entries in the POWL tables can't be updated once they are created.

To update the customized POWL, first we need to select and delete the inconsistent entries then add them again as new entries.

This has to be done in the transaction POWL_TYPER:

Go to the transaction POWL_TYPER and do the following.

1. Select and delete entries with the application SOLMAN_NOTIF_MGMT.

2. Click on New Entries button and create the following entries.

      a) Type: DSWP_NA_POWLTYP_EXTERNAL Application: SOLMAN_NOTIF_MGMT
      b) Type: DSWP_NA_POWTYP_MANAGEDSYSBPS Application: SOLMAN_NOTIF_MGMT
      c) Type: DSWP_NA_POWLTYP_MANAGEDSYSUSRS Application: SOLMAN_NOTIF_MGMT
      d) Type: DSWP_NA_POWLTYP_SOLMANSYSBPS Application: SOLMAN_NOTIF_MGMT
      e) Type:DSWP_NA_POWLTYP_SOLMANSYSUSERS Application: SOLMAN_NOTIF_MGMT

Ensure there are no duplicates exist.

In order to reflect the changes on the screen you will have to  clear the POWL cache by running a report called POWL_D01 with APPID: SOLMAN_NOTIF_MGMT

In case if role assigenment is correct and still facing this issue then you will have to check for the authorization object CA_POWL has application ID SOLMAN_NOTIF_MGMT or check whether all the authorization values are maintained properly for the role that contain CA_POWL.