1.Was the issue occurred recently? Were you able to remote to AWS instance via RDC for mac before?
2.Kindly confirm you have set up well to enable the AWS instance to accept the remote session. Here is the reference for common cases of computers running with windows OS.
Note: You can't connect to a PC that's asleep or hibernating, so make sure the settings for sleep and hibernation on the remote PC are set to Never. (Hibernation isn't available on all PCs.)
3.Per checking, here are the steps for configurating the AWS instance before starting remote connection:
Connect to your Windows instance using RDP
4.You could also try the latest version of RDC for mac Beta version with new features.
Hope this helps and please help to accept as Answer if the response is useful.