Per IBM, the Spectrum Protect server is supported on Ubuntu LTS 14, 16, 18, and 20 (aka 2014.04, 2016.04, etc.) 

https://www.ibm.com/support/pages/overview-ibm-spectrum-protect-supported-operating-systems

However, Operations Center (web GUI) is not supported on Ubuntu, only RHEL and SLES.

https://www.ibm.com/support/pages/ibm-spectrum-protect-operations-center-software-and-hardware-requirements

./install.sh -c
Validating package prerequisites...
=====> IBM Installation Manager> Update> Prerequisites
Validation results:
* [ERROR] IBM Spectrum Protect Operations Center 8.1.12000.20210326_0723 contains validation errors.
1. ERROR: The operating system on which you are installing the product is not supported. For more information, see http://www.ibm.com/support/docview.wss?uid=swg21243309.

Enter the number of the error or warning message above to view more details.

To skipp the OS and platform checks, and convert the ERROR into WARNING:

./install.sh -c -vmargs "-DBYPASS_TSM_REQ_CHECKS=true"
Validation results:
* [WARNING] IBM Spectrum Protect Operations Center 8.1.12000.20210326_0723 contains validation warning.
1. WARNING: The operating system on which you are installing the product is not supported. For more information, see http://www.ibm.com/support/docview.wss?uid=swg21243309.

Enter the number of the error or warning message above to view more details.

I recommend ONLY install/update Operations Center with this, and then exit and go back in normally to make sure the other filesets validate okay.