CAPsMAN instructions result in "Invalid value in Master"

After any change, share config. This is the only way to provide proper help.

Furthermore, there is pretty good documentation including examples:
https://help.mikrotik.com/docs/spaces/ROS/pages/224559120/WiFi#WiFi-CAPsMAN-CAPVLANconfigurationexample:

Documentation on VLAN is there as well, but I prefer to use this topic as guide:
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1

I chose to use trunk ports on the accesspoints as well. And have a dedicated mgt vlan where the CAPsMAN and CAPS’s are communicating over.