Hello Casa_prata,
If using column layout check your settings to make sure 'Flag' is selected. Use column layout in Mail on Mac
If you're not using column layout, check out the steps below from the guide here: Flag emails in Mail on Mac to help you view your flagged messages.
"Show flagged emails
- In the Mail app on your Mac, click the arrow next to Flagged in the Mail sidebar or the Favorites bar, then choose a flag.
- Only messages marked with that flag are shown in the message list.
- To show all messages in the Inbox again, click Inbox in the Mail sidebar or the Favorites bar.
You can search for flagged messages. Type a flag name in the search field, then select the suggestion in the Flags category. Or type “flag,” then choose the suggestion in the Status category."
The section "Flag messages" in the article here can also help: Organize email on your Mac
Cheers!