You need 2 licenses of SQL Response if you wish to install the tool on a cluster, as you need a license for each node.
If you believe you have enough licenses to cover this node, please can you send an e-mail to support(at)redgate.com referencing this forum post along with your license information?
Red Gate Software Ltd
+44 (0)870 160 0037 ext. 8569
1 866 RED GATE ext. 8569