Hello, @lionking62!
We are planning to support Ubuntu 20.04 in NBR v10.4. The release is planned for July 2021.
Please find the extended Linux OS support list for it: Ubuntu 20.04, RHEL7.6/7.7/8.0/8.1/8.2, SLES15/CentOS 8
As for now, you can upgrade OS Ubuntu to v18.04 which is currently supported as well.
The data sheet for VMware Backup and Replication 10.3 and the Supported Platforms section of the User Guide both list "Ubuntu 16.04–18.04 Server (x64)". That's not to say it won't work on the later version, but you should speak with them directly about your support options.