BMXP342020 supports Maximum 36 expert channels included CPU application channels i.e. modbus and each ERT module consume 4 channels per module.
So if 9 BMXERT1604 is being installed then it will consume =9x4 channels =36channels
And one channel will be occupied by modbus port on CPU.
Hence the maximum limit of expert channels will be exceeding(i.e. more than 36 Channels).
So replace the CPU to BMXP3420302 v2.50 as it is not having any embedded modbus port.
So if 9 BMXERT1604 is being installed then it will consume =9x4 channels =36channels
And one channel will be occupied by modbus port on CPU.
Hence the maximum limit of expert channels will be exceeding(i.e. more than 36 Channels).
So replace the CPU to BMXP3420302 v2.50 as it is not having any embedded modbus port.