getting an error "The userId parameter that is passed to the emailUser method cannot be set to the value <mail>. It must be of type uuid."

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As explained in the wix-users.emailUser() documentation:
You can only send the email to the currently logged-in member. You can get that member’s ID using the id property of the currentUser .

Your call to emailUser should look something like this:

let userId = wixUsers.currentUser.id;
wixUsers.emailUser("SYnlqrp", userId)

Note that the same thing applies to emailContact().

If you want to send an email to a different user than the one that is currently logged in, you will need to use the backend APIs: wix-users-backend, and wix-crm-backend.