why *ngIf doesnt'work with ng-template?

Read the doc here https://angular.io/guide/structural-directives especially for

<div *ngIf="hero" >{{hero.name}}</div>

The asterisk is "syntactic sugar" for something a bit more complicated. Internally, Angular desugars it in two stages. First, it translates the *ngIf="..." into a template attribute, template="ngIf ...", like this.

<div template="ngIf hero">{{hero.name}}</div>

Then it translates the template attribute into a element, wrapped around the host element, like this.

<ng-template [ngIf]="hero"> <div>{{hero.name}}</div></ng-template>

• The *ngIf directive moved to the element where it became a property binding,[ngIf].
• The rest of the , including its class attribute, moved inside the element.

So for it we have ng-container

 <ng-container *ngIf="seat.section">
    Section {{seat.section}} ,
 </ng-container>

or use span or div or regular html tag.

 <span *ngIf="seat.section">
    Section {{seat.section}} ,
 </span>

or if you still want to use ng-template (not recommended)

<ng-template [ngIf]="seat.section">
  Section {{seat.section}} ,
</ng-template>