Accounts that exist on your cPanel & WHM server, but do not exist in WHMCS are considered unsynchronized accounts.
The fastest way to synchronize your accounts is to use WHMCS' Server Sync tool.
Alternatively, you can authenticate into your cPanel & WHM server using Terminal and enter the following command to see a list of unsynchronized accounts:
for accounts in $(whmapi1 listaccts |awk '/user:/ {print $2}'); do apicheck=$(whmapi1 list_integration_links user="${accounts}" | grep 'app: WHMCS'); if [[ -z "${apicheck}" ]]; then echo "${accounts}"; fi; done
You can learn more about integrating WHMCS into your cPanel & WHM experience here: