Hello @Jane ,
Thank you for reaching out to Microsoft Q&A platform. Happy to answer your question.
The coginitive services can be created under azure free trial. By this, I can confirm that custom vision resources can be created under free account
Reference : custom-vision-service
Listed as free service under AI at this document link talking about all the azure free services with trial account free-services
Make sure that the "Microsoft.CognitiveServices" is registered under resource providers on your subscription. To check and validate, navigate to global search bar on portal.azure.com-> look for subscriptions->Resource Providers-> Microsoft.CognitiveServices
Also check if Microsoft.MachineLearning is registered too.
Once checked, please check for custome vision service, or use this link ms.portal.azure.com
Please "Accept as Answer" and Upvote if any of above helped so that, it can help others in community looking for remediation for the similar issues.