Carry out the following steps to create a custom Reason Lost field on the opportunity object:

1. Click on the Setup gear icon in the top right-hand corner of the main Home page, as shown in the following screenshot:

2. Click the Setup option, as shown in the following screenshot:

3. Navigate to the Opportunity customization setup page by clicking the following: Objects and Fields | Object Manager | Opportunity | Fields & Relationships.

Locate the Fields & Relationships section on the right of the page.

4. Click on New.

You will be presented with the Step 1. Choose the field type page.

5. Choose Picklist from the Data Type options.
6. Click on Next.

You will be presented with the Step 2. Enter the details page.

7. Type Reason Lost in the Field Label textbox.
8. Accept the default Reason_Lost value in the name field.
9. Choose the Enter values, with each value separated by a new line option.
10. Paste in, or enter, these example values into the textbox:

No Budget 
Missing Product Features 
Better Price 
Cost / Value

11. Optionally, enter details in the Description field.
12. Optionally, enter details in the Help Text field.
13. Click on Next, as shown in the following screenshot:

You will be presented with the Step 3. Establish field-level security page.

14. Select the profiles to which you want to grant edit access to this field via field-level security. The field will be hidden from all profiles if you do not add it to field-level security.

15. Click on Next.

You will be presented with the Step 4. Add to page layouts page.

16. Select the page layouts that should be included this field. The field will be added as the last field in the first two-column section of these page layouts. The field will not appear on any pages if you do not select a layout.
17. Finally, click on Save.