How to “Lock Excel File”: You must enter a password before opening the file.

1. The first step is to click on File.

2. Then click to select info.

3. Select Protrct Workbook and select Encrypt with Password.

4. Then enter your password.

5. And enter the verification code again

6. Then exit the program again to check whether it is finished or not.

Note : If finished, you will be prompted to enter the code and you are done.

Credit : การสอนวิธีล็อกไฟล์ Excel ด้วยรหัสผ่าน (คู่มือผู้เริ่มต้นใช้งาน) (altitudetvm.com)