Cannot run enable-remotemailbox for exising user in AD within a hybrid envrironment

[yikes123]

I have an on prem Exchange 2019 with a hybrid setup.  When I create a new user in AD, I can run enable-remotemailbox [identity] -primarysmtpaddress [email address removed for privacy reasons] -remoteroutingaddress [email address removed for privacy reasons]

However, I have some users what are just in AD with mail enabled that I would like to migrate to Exchange online via hybrid.  When I run the above command I get:

This task does not support recipients of this type. The specified recipient [me] is
of type UserMailbox. Please make sure that this recipient matches the required recipient type for this task.

In the exchange properties, it is set mailbox type 'user', and under email address I do not see the box/option 'set as remote routing address:'.

What am I doing wrong?  I can currently access the account's email through webmail, so I know there is email working.

[#2]

If the on-premises AD object is of the type UserMailbox, it generally means they already have a mailbox, and that mailbox is located on-premises, rather than in Exchange Online. You'll need to migrate the mailbox from on-prem Exchange to EXO. The other option is to delete the on-premises mailbox and then enable the remote mailbox (data will be lost from the on-prem mailbox if you do this)