HDCP error with old av receiver

Hello. I'm trying to set up the Roku Express 4k + to my friend's old Yamaha RX-V640 avr. There are no HDMI ports so we are using a HDMI to yPbPr+R/L converter. I had to boot up the Roku on a different TV and lower the resolution to 720p to get it to work without getting connection errors. It works fine now, both picture and sound come through, however, when we go to play a video off youtube or Netflix we can see the video for about half a second then we're met with an HDCP error detected. My question is could this just be a problem with the receiver being too old and not supporting HDMI 2.0 or could it be something wrong with the HDMI converter?