A new version of the AWS provider for Terraform, 3.14.0, was released on November 5th which introduced a regression that was causing builds to fail with an error like the one below. This regression was isolated to the aws_cloudwatch_event_target resource for the AWS provider. The regression was due to the resource always pushing a BusName argument even if one was not defined.
A patch was merged today on November 6th to remedy this issue and has been released in version 3.14.1 of the AWS provider. This patch changed the resource in question by ensuring that if the optional event_bus_name argument was not specified in the resource block, it would not try to pass an empty string to the API call which was causing the failure. To ensure that the AWS provider version you are using does not experience this regression on this resource, ensure that any provider pinning on the AWS provider either skips the 3.14.0 minor version or will pick up the 3.14.1 patch version as well.