I can't say why this is happening, but the 'category, subcategory, tags' are optional fields.
First I would make sure your user you are looking at the dash has alert manager permissions.
Second I like to use the "Add to Triggered Alerts" option from alert actions along with the "Alert Manager" option. This way you can verify your alert is working as expected with the splunk alerting subsystem outside of alert manager.
Once you have alerts showing in "Triggered Alerts" you should see them in the dashboard context.
you can also check the index you assigned at install for any incidents, and the kvstore too with
| `all_alerts`
to see if there is anything there
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