Use the ngx-charts version 18 or below.

The scheme type [scheme]="colorScheme", is asking for a string but the type you are providing is an object.

The latest version 19 or above as scheme type is of string | color.

ngx-charts version 19 or above accepts only types string or Color (from here), and to expand on another answer:

• String examples include "cool", "natural", "vivid", etc from the preset color schemes in this file.
• Color example to set your own custom scheme:

import { Color, ScaleType } from '@swimlane/ngx-charts';
export class foo {
  colorScheme: Color = {
    name: 'myScheme',
    selectable: true,
    group: ScaleType.Ordinal,
    domain: ['#f00', '#0f0', '#0ff'],
  };
}

<ngx-charts-area-chart 
 ...
 [scheme]="colorScheme"
></ngx-charts-area-chart>