PHP 8.1 is supported starting from 4.16.1. Ivan has provided the details on this matter:
PHP 8.0 is supported starting with CS-Cart/Multi-Vendor 4.15.1 as you have already mentioned. Please make sure that all the necessary PHP extensions are installed for this version of PHP.
This information will most likely be added once this version of PHP has been fully tested. Since its compatibility hasn’t been fully tested yet, we won’t add it to the list of supported versions on the above documentation page.