VM's Status in client area of WHMCS is unauthorized like this:
Missing custom fields entries in WHMCS. As described in SolusVM documentation custom fields are mandatory.
Configure custom fields my following the steps in the documentation or using the following article as a reference: How to create a product in WHMCS with solusvmpro module?
For all new VMs it will work properly afterwards.
For already existing ones, update vserverid in WHMCS by taking the corresponding value from SolusVM > Virtual Servers > List Virtual Servers.