diff --git a/moto/iam/models.py b/moto/iam/models.py
index 81c7d09ca..a7035c445 100644
--- a/moto/iam/models.py
+++ b/moto/iam/models.py
@@ -735,7 +735,9 @@ class IAMBackend(BaseBackend):
         )
         if policy.arn in self.managed_policies:
             raise EntityAlreadyExists(
-                f"A policy called {policy_name} already exists. Duplicate names are not allowed."
+                "A policy called {} already exists. Duplicate names are not allowed.".format(
+                    policy_name
+                )
             )
         self.managed_policies[policy.arn] = policy
         return policy