First let me clear one thing I have gone though https://github.com/ghiscoding/angular-slickgrid/wiki/HOWTO---Step-by-Step and followed each step which is mandatory required. Anyhow let me show you what step i have followed
- 
Installed angular slickGrid npm i angular-slickgrid
 
- 
Added required css and js in angular.json
 
    "node_modules/bootstrap/dist/css/bootstrap.css",
    "node_modules/font-awesome/css/font-awesome.css",
    "node_modules/flatpickr/dist/flatpickr.css",
    "node_modules/angular-slickgrid/lib/multiple-select/multiple-select.css",
    "styles.css"
],
"scripts": [
    "node_modules/jquery/dist/jquery.js",
    "node_modules/jquery-ui-dist/jquery-ui.min.js",
    "node_modules/slickgrid/lib/jquery.event.drag-2.3.0.js",
    "node_modules/bootstrap/dist/js/bootstrap.js",
    "node_modules/angular-slickgrid/lib/multiple-select/multiple-select.js"
],
- 
imported AngularSlickgridModule in app.module.ts properly
imports: [ AngularSlickgridModule.forRoot() ]
 
- 
already i have installed required package for angular slickgrid
package.json
 
  "@angular/animations": "^9.1.9",
  "@angular/cdk": "^9.2.4",
  "@angular/compiler": "^9.1.9",
  "@angular/material": "^9.2.4",
  "@angular/platform-browser": "^9.1.9",
  "@angular/platform-browser-dynamic": "^9.1.9",
  "@angular/router": "^9.1.9",
  "bootstrap": "^4.4.1",
  "font-awesome": "^4.7.0",
  "@types/jquery": "^3.3.38",
  "jquery": "^3.3.1",
  "@ngx-translate/core": "^12.1.2",
  "@ng-bootstrap/ng-bootstrap": "^6.1.0",