Hi Ravinder,
It's not that Rollup fields can't sum custom fields, but it can only apply filters to the source entity or related entities, simple fields or non-complex calculated fields.
More information: https://docs.microsoft.com/en-us/dynamics365/customerengagement/on-premises/customize/define-rollup-fields#rollup-field-considerations
The TCV field is a complex calculated field here, which is why you cannot find the TCV field in the Rollup field. We have to recreate the TCV field to make it available for Rollup field.
So I would recommend you create a simple field for TCV, and use Power Automate to calculate it, instead of use calcultated field.
Step 1. Create a new simple field.

Step 2. Create the first flow - automate cloud flow.


mul(outputs('Get_a_row_by_ID')?['body/estimatedvalue'],triggerOutputs()?['body/new_dealterm'])
After saving this flow, the TCV field value will be calculated when "deal term" field changes.
Step 3. Create the second flow - automate cloud flow.
Overview:

Details:



mul(outputs('Get_a_row_by_ID')?['body/new_dealterm'],triggerOutputs()?['body/estimatedvalue'])
After saving this flow, the TCV field value will be calculated when "Estimated revenue" field changes.
Step 4. Create an instant cloud flow.
Overview:

Details:




After runing this flow, the TCV field value of all existing Opportunity Products will be calculated.
Step 5. Create a Rollup field using the new simple TCV field.