Why my hard disk shows error like this POST ERROR

Yemam, the POST (Power-on Self Test) is a system that checks for errors whenever the computer powers on, usually during BIOS startup. SMART (Self-Monitoring, Analysis, and Reporting Technology) is a similar system used in most HDDs (Hard Disk Drives, a.k.a. hard drives) and SSDs (Solid State Drives) that checks for errors in the drive, as well as performing diagnostics, measuring drive reliability, and even predicting imminent drive failure. SMART errors can be caused by a wide variety of issues and factors, including (but not limited to):

• a high Read Error Rate
• a high Reallocated Sectors Count
• a high Spin Retry Count
• an End-to-End error / IOEDC
• a large amount of Reported Uncorrectable Errors
• Command Timeouts
• a high Reallocation Event Count
• a high Current Pending Sector Count
• Current Pending Sector Count
• a high (Offline) Uncorrectable Sector Count
• a high Soft Read Error Rate

For now, there are only a few things you can do:

• Check if the error message provides a specific error code
• Try plugging the drive into a different computer, and running a SMART self-test on it using a program such as Smartmontools (it doesn't really matter what SMART tool you use, I just recommended Smartmontools because it's easy to install, free, and allows for manual testing. You can find a comparison chart of SMART tools here).