@Nairi2001 You may have exceeded the storage sync limit. If so, you will see a warning that the most recently added items cannot be saved. To fix this, open the Note Sidebar Options page -> Advanced Options tab, and switch the storage setting from "Sync Storage" to "Local Storage". This will give you unlimited space to write and save notes directly on your device. However, keep in mind that these notes will not sync across your other devices.
If you need any further assistance, please do not hesitate to use the official support at https://www.stefanvd.net/support
Thanks,