Dear M365 Admin4,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, it seems that you are having concern with OneDrive, user cannot sync OneDrive shared folder. For the situation you encountered, we do understand the inconvenience caused and feel very sorry about it.
To help you better, we' d like to confirm how is the folder shared. Is it shared via Anyone link or other way?
Meanwhile, we've done a quick test on our side, we can sync the folder by accessing the shared link generated by "People you choose", and we cannot see the sync button when we access the link generated by "Anyone" link on our side.
For your reference: Compartir archivos y carpetas de OneDrive - Soporte técnico de Microsoft

Given this, we'd recommendy you share the folder by generating a "People you choose" link again to check the outcome. Meanwhile, please check if Agregar y sincronizar carpetas compartidas a OneDrive para el hogar - Soporte técnico de Microsoft can help.
Welcome to post back if any updates. We look forward to your reply.
If the understanding above is not consistent with yours, you can also post back and point that.
Your understanding and patience will be highly appreciated. We hope that you are keeping safe and well!
Thanks for your cooperation.
Sincerely,
Rhoda | Microsoft Community Moderator