How to decrypt windows administrator password in terraform?

The password is encrypted using the key_pair you specified when launching the instance, you still need to use it to decrypt as password_data is still just the base64 encoded encrypted password data.

You should use ${rsadecrypt(self.password_data,file("/path/to/private_key.pem"))}

This is for good reason. You really don't want just a base64 encoded password floating around in state.

Short version: You are missing the second argument in the interpolation function.