external backup for time machine

Maybe your PassPort is not properly formatted. The Time Machine volume should be formatted MacOS Extended (Journaled) with a GUID Partition Map scheme. Use Disk Utility (in the Applications/ Utilities Folder) to format your Time Machine drive. Formatting will erase any data that is on the drive. So if you have data you want to save, copy it to another drive or location. You can also Partition your drive if you only want to use a portion of it for Time Machine.

Types of disks you can use with Time Machine on Mac - Apple Support

Connect a new backup disk to your Mac - Apple Support

If Time Machine on your Mac can’t find your backup disk - Apple Support

Welcome to Disk Utility on Mac - Apple Support

Hi,

I just wonder how to connect external drive to iMac, connect directly, or hub or adapter?

You would better connect the drive directly or adapter since external drive need electronic power supply via USB-C or USB3 bus.