target cannot be convertible to afps error in macos high sierra also it says helper tool crashed . How to reinstall Mac OS High Sierra ?

Is your drive an SSD or an HDD? When you run the installer for an HDD be sure to deselect an option to convert the drive from HFS+ to APFS. Is your startup volume a single drive or a RAID array? Also, see How to Skip Converting to APFS When Installing macOS High Sierra.

My hard disk is an SSD but I don’t want to convert it into APFS

A boot SSD must be APFS. There is no opting out of that requirement.

Prepare for APFS in macOS High Sierra - Apple Support

I imagine there may be ways to subvert that posted on the internet, but they certainly are not guaranteed to work.

I don't think you succeeded in not converting your SSD to APFS but there was something wrong with the SSD, and it couldn't be converted. Now you will have to reformat your SSD. If you wish to have the SSD formatted HFS+ instead of APFS then see the following: How to Skip Converting to APFS When Installing macOS High Sierra and Is It Possible Not to Convert to APFS When Upgrading to High Sierra?.

Since you appear to have a functioning Recovery HD then you can do the following:

Install El Capitan or Later from Scratch

If possible backup your files.

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed.

1. All SATA drives are compatible. The vast majority of bare drives will come formatted for Windows. To use on the Mac they must be partitioned to use the GUID schema and be formatted Mac OS Extended, Journaled. In the case of an SSD, they can also be formatted APFS if you are running High Sierra.
2. You cannot backup while in recovery mode unless you know how to use the Terminal and the command line. macOS cannot write to an NTFS formatted drive. It can only read from it.
3. If you install High Sierra on to an SSD it will convert it to APFS if it is already formatted HFS+. You can opt not to format APFS: How to Skip Converting to APFS When Installing macOS High Sierra and Is It Possible Not to Convert to APFS When Upgrading to High Sierra?.

Did you also convert the SSD to apfs or is it still in HFS+?

As Kappy explained you can choose not to convert to APFS on an SSD when installing HighSierra.

I am curious to know how you succeeded to succes.

target cannot be convertible to afps error in macos high sierra also it says helper tool crashed . How to reinstall Mac OS High Sierra ?

Glad to help. Have a great weekend. 😀