Do I require RDS CAL license to access Virtual Personal Desktop via RD Web

Current setup:
1. A Virtual Personal Desktop with an OS for Windows 7 Pro is set up
2. Users connect to this VPD via RD Web

Issues:
User received a prompt when selecting the My Desktop icon in RD Web as follows:
“The remote session was disconnected because there are no Remote Desktop client access licenses available for this computer. Please contact the server administrator.”

Question:
So do I require to get a Remote Desktop CAL license for this VPD.
I assumed that accessing the VPD via RD web do not require any remote desktop license since it can be configured to be accessible from external connection.

Please advise.

Also we did not purchase RDS CAL license for this Virtual Personal Desktop from the start.