The SharePoint Framework solutions (SPFx) are most of the times deployed at the Tenant APP Catalog and external users don’t have access to that resource by default. They will see an error message and that is related with Permission of App Catalog. You need to make the solutions files available for yours External Users and because of that, you need to configure your SharePoint Tenant to grant access for those users.
The first step is adding the Everyone group to your App Catalog site. I would recommend providing them with reading permission by adding them to the Visitor group.
By default, the Everyone group is deactivated (link at references for more information), so you need to activate this group at the tenant level by running the below command. Note that you might need to wait for a few minutes after executing this command:
Connect-PnPOnline -URL https://contoso-admin.sharepoint.com Set-PnPTenant -ShowEveryoneClaim $true
After you wait a few minutes go to your Site Permissions of App Catalog, select the Visitors Group and add the Everyone group. After a few minutes, so all the permissions are propagated, the SharePoint Solutions deployed at the App Catalog will start to be visible to all the external users as well.
Now everyone that access a site can see your custom solutions without any error. I want to reference another article from Laura Kokkarinen which can have other valid solution for this scenario, link at the reference section. Another solution to overcome this scenario is to create an App Catalog for the SharePoint site that you’re allowing external users.