Why do I get a message that mail drop is not available "try again later"

Apple Mail Drop is a service which allows you to send email attachments that are too large to send normally as a regular email attachment. When your email account sending size limit is exceeded, Mail Drop is used. If you have Mail Drop enabled for the particular email account a link is inserted to the email and the file cloud hosted on iCloud. The attachments do not deduct from your iCloud storage and the file will only be available for a 30 day time period. There is a 1TB limit on stored Mail Drop files in iCloud per user. The maximum size of Mail Drop attachments is 5GB. There are limits on how many people can download a Mail Dropped file but Apple didn't document it. For example, if you emailed hundreds of people and they all keep clicking the Mail Drop link, eventually, Apple will restrict the file to prevent a potential DDoS attack. A flood of network traffic intended to overwhelm the servers.

Mail Drop is listed as a service on the Apple Status page so if there is an outage with Mail Drop you will see it listed here alongside all the other Apple service status. https://www.apple.com/support/systemstatus/

Another possibility is that you are on a managed network that blocks connectivity to the Mail Drop service in iCloud. Such as a work or school network.

As mentioned in the beginning, it could also be you exceeded the 1TB storage limit or have been using the service too heavily.

Alternatives to Mail Drop would include sharing the file via a cloud provider such as iCloud, OneDrive, Google Drive, Dropbox, etc.