This is a known cosmetic issue which does not affect vSAN over RDMA functionality.
"PFC is not Enabled" warning seen on hosts enabled for vSAN over RDMA and using Intel(R) Ethernet Network Adapter E810-XXV-2 with icen driver version 1.5.5.0 and irdman driver 1.3.3.7"
vSphere UI, vSAN cluster object: Monitor -> vSAN -> Skyline Health displays "RDMA Configuration Health" warning for each vmnic used for vSAN over RDMA.

Health check displays column values for each vmnic as follows:
- RDMA Mode: IEEE
- PFC Enabled: No
- PFC value: --
- Issues: The PFC is not enabled
On an ESXi node, PFC Enabled reported as 'false' while actually enabled.
Example of configuration output for a vSAN over RDMA vmnic:
[root@esxi-host:~] esxcli network nic dcb status get -n vmnic6
Nic Name: vmnic6
Mode: 3 - IEEE Mode
Enabled: true
Capabilities:
Priority Group: true
Priority Flow Control: true
PG Traffic Classes: 8
PFC Traffic Classes: 8
PFC Enabled: false <---------------------------------- should display 'true'
PFC Configuration: 0 0 0 0 0 0 0 0 <------------------ should display '0 0 0 1 0 0 0 0'
IEEE ETS Configuration:
Willing Bit In ETS Config TLV: 0
Supported Capacity: 8
Credit Based Shaper ETS Algorithm Supported: 0x0
TX Bandwidth Per TC: 50 50 0 0 0 0 0 0
RX Bandwidth Per TC: 50 50 0 0 0 0 0 0
TSA Assignment Table Per TC: 2 2 0 0 0 0 0 0
Priority Assignment Per TC: 1 1 1 0 1 1 1 1
Recommended TC Bandwidth Per TC: 50 50 0 0 0 0 0 0
Recommended TSA Assignment Per TC: 2 2 0 0 0 0 0 0
Recommended Priority Assignment Per TC: 1 1 1 0 1 1 1 1
IEEE PFC Configuration:
Number Of Traffic Classes: 8
PFC Configuration: 0 0 0 1 0 0 0 0
Macsec Bypass Capability Is Enabled: 0
Round Trip Propagation Delay Of Link: 0
Sent PFC Frames: 0 0 0 0 0 0 0 0
Received PFC Frames: 0 0 0 0 0 0 0 0
DCB Apps:
App Type: L2 Ethertype
Protocol ID: 0x8906
User Priority: 0x3
App Type: L2 Ethertype
Protocol ID: 0x0
User Priority: 0x0