This issue occurs when a vApp Template is captured to a Catalog and has not had the TAG WITH COMPUTE POLICIES > Modifiable option selected.
When vApps are deployed from a vApp Template where the modifiable option is not set they receive a setting to mark the Placement or Sizing Policy to not be modifiable.
More details on this option are available in the Cloud Director documentation, Assign a VM Placement Policy and a VM Sizing Policy to a vApp Template Using Your VMware Cloud Director Tenant Portal.
To resolve the issue, you will have to change the modifiable option on the captured vApp Template.
Details are in the Cloud Director documentation, Assign a VM Placement Policy and a VM Sizing Policy to a vApp Template Using Your VMware Cloud Director Tenant Portal.
For example: