i have a external but i can't reformat so how do i use time machine

Why can't you format it?

Drive Preparation - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list. 
3. Click on the Erase button in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
5. If desired, you can change the volume's Desktop name.
6. Click on the Apply/Erase button and click on the Done button, when it is activated.
7. Quit Disk Utility.

If it is not formatted HFS+, then you can't use it for Time Machine. You need a file System that supports hard links to folders. HFS+ is one of the few file systems that supports that.

Why can't you reformat it?