Whenever defining a Web Part property in the Web Part manifest the same property should be defined in Web Part's code so that it's configurable through the property pane.

WeatherWebPart.manifest.json

{
  "$schema": "../../../node_modules/@microsoft/sp-module-interfaces/lib/manifestSchemas/jsonSchemas/clientSideComponentManifestSchema.json",

  "id": "8908df13-a965-4445-9c48-ecec77c44be4",
  "alias": "WeatherWebPart",
  "componentType": "WebPart",
  "version": "0.0.1",
  "manifestVersion": 2,

  "preconfiguredEntries": [{
    "groupId": "8908df13-a965-4445-9c48-ecec77c44be4",
    "group": { "default": "Default" },
    "title": { "default": "Weather" },
    "description": { "default": "Shows weather for the given place" },
    "officeFabricIconFontName": "Sunny",
    "properties": {
      "location": ""
    }
  }]
}

WeatherWebPart.ts

import { Version } from '@microsoft/sp-core-library';
import {
  BaseClientSideWebPart,
  IPropertyPaneConfiguration,
  PropertyPaneTextField
} from '@microsoft/sp-webpart-base';
import { escape } from '@microsoft/sp-lodash-subset';

import styles from './Weather.module.scss';
import * as strings from 'weatherStrings';
import { IWeatherWebPartProps } from './IWeatherWebPartProps';

export default class WeatherWebPart extends BaseClientSideWebPart<IWeatherWebPartProps> {
  public render(): void {
    // omitted for brevity
  }

  protected getPropertyPaneConfiguration(): IPropertyPaneConfiguration {
    return {
      pages: [
        {
          header: {
            description: strings.PropertyPaneDescription
          },
          groups: [
            {
              groupName: strings.DataGroupName,
              groupFields: [
                PropertyPaneTextField('location', {
                  label: strings.LocationFieldName
                })
              ]
            }
          ]
        }
      ]
    };
  }
}

SharePoint Code Analysis Framework, v6.9.1.2001

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