Your IP address is not allowed access to the Web Monitor as per configuration settings.
Please consult http-access directive in PMTA User Guide.
For reference, the directive for administrative access from your current IP address that needs to be added to /etc/pmta/config would be:
http-access <your IP address> admin
To find the IP address you used to access the Web Monitor from, please look in the Web Monitor log file located at: /var/log/pmta/pmtahttp.log