This article helps to resolve an issue with "Add-in is not trusted, you should trust it before you can use it" when navigating to installed add-in home page.

Mostly this message appears for new tenants. It means, that add-in you want to use to deploy your Solution doesn't have permissions to the Site where it's installed (even if you trust it during installation process). 

skybow Add-in deployment use ACS (Azure Access Control) app-only access token to get permissions only for current site to start deployment process and for new tenants this access model is disabled by default.

To check it, you could run simple PowerShell Command: Get-SPOTenant and find then DisableCustomAppAuthentication property in a result:

$orgName="<name of your Office 365 organization, example: contosotoycompany>" 
Connect-SPOService -Url https://$orgName-admin.sharepoint.com -Credential Get-credential
Get-SPOTenant

  Connect-SPOservice - will ask for Credentials to connect to SharePoint Online by Url to admin center. Please use Global Admin credentials.

  Get-SPOtenant - will get all tenant properties and it's values.

In a result screenshot above value of DisableCustomAppAuthentication property is set to True which means that add-ins with ACS (Azure Access Control) app-only access token model disabled.

To change this behavior you can use another simple PowerShell command set-spotenant -DisableCustomAppAuthentication $false'. This will change the value for the property and should be done only once.

$orgName="<name of your Office 365 organization, example: contosotoycompany>" 
Connect-SPOService -Url https://$orgName-admin.sharepoint.com -Credential Get-credential
set-spotenant -DisableCustomAppAuthentication $false

  Connect-SPOservice - will ask for Credentials to connect to SharePoint Online by Url to admin center. Please use Global Admin credentialsю.

  set-spotenant - will set DisableCustomAppAuthentication tenant property to false. This will allow to use add-ins with ACS app-only access tokens.

Note: Install latest SharePoint Online Management Shell to get correct results.

Note: Global admin credential should be used to connect to SharePoint Online and get/set DisableCustomAppAuthentication property

After set-spotenant command is executed, please login to your site again and try to re-trust installed add-in for deployment again.

Now you should be able to navigate to home page and start/check your deployment.