In Trados Studio,  when trying to use NMT, you receive the following error message:

An error has occurred while using the translation provider SDL Machine Translation:
Number of allowable characters exceeded. Check your subscription in File > Language Cloud.

If you are interested in purchasing a subscription package with more characters, please get in touch: https://www.sdltrados.com/contact/

The Free subscription is renewable each month, split into 500,000 characters/month. But, this is a yearly quota and you may have spent all the characters. 
The message informs you that you have reached this quota. You can check the reports by going to https://translate.sdl.com/reports/dashboard

See this detailed article on how to see the NMT usage in your Machine Translation Cloud account:

How to check Machine Translation usage for the Language Cloud Free subscriptions or for the Language Weaver Cloud subscriptions