Why can I not send from my @me.com address from the icloud mail site?

Greetings gvnshtn,

It sounds as though you're wanting to send emails from your me.com email address via the iCloud website. To confirm, are you wanting to set the me.com email as your default email address? If so, you can do so via the steps listed below. You can also find these steps outlined in the following article.

1. "In Mail on iCloud.com, click  at the top of the Mailboxes list, then choose Preferences.
2. Click Composing, click the “Set a default address” pop-up menu, then choose an email address.
3. In the list of addresses below, select the ones you want to be able to send from in iCloud Mail.
4. Click Done."

Choose a default email address to send mail from on iCloud.com - Apple Support

If you're wanting to keep the default email but change which email address is used to send the message, you can tap the email in the "From" field, and choose the desired email address.

Have a great day!