Installation: Roll up Summary Field Limit preventing installation

Issue:

During installation, it is possible that some larger and older Organizations will hit their limit of available Salesforce Field relationships. The errors will only present themselves one at a time, so it is possible that you will need to resolve multiple issues in a single install. (Unlikely, but possible.)

Error Message
(Opportunity.CPDealCount__c) Custom Roll-up Summary Field Exceeded
Unable to create a new custom roll-up summary field because it exceeds the maximum allowed: 25

ANSWER:
Remove unused Summary Field
or
Increase the limit to 40.

The 'soft' limit is 25. If you are currently using 25 custom roll-up fields, then you can submit a request to Salesforce to increase the amount to the 'hard' limit of 40.
Salesforce can usually do this in 24 hours or less.

If you are already at 40 custom roll-up fields on the Opportunity Object, you must remove an unused or deprecated Custom Roll-up field to install ClosePlan.

Please review your Custom Roll-up fields on the Opportunity Object to find one to free up.

Note: The Field you free up to solve this issue MUST be a custom roll-up field.