There is no "Login to Plesk Control Panel" button for the reseller, which credentials are used to integrate WHMCS and Plesk.
The following error appears in WHMCS client area:
Error code: 1006. Error message: Permission denied.
Such behavior is linked with a software issue #PPPM-2719, which has been fixed in MU #49.
Update Plesk and Plesk module in WHMCS to the latest versions.