If the app has no position for your yacht, Map still opens, but it opens zoomed out with no pin on it, the chart is drawn in grey, and a warning sits in the middle: the location is not available because the vessel's MMSI was not added, or because no AIS data is available at the moment.
Choosing Dismiss clears the warning and returns the chart to its normal colours, so you can pan and zoom around it. It does not fix anything — it just gets the message out of the way.
There are two quite different causes, and it is worth telling them apart.
The MMSI is missing or not a valid one
The MMSI is the nine-digit identity that ties your yacht to its AIS broadcasts. If the field is empty, holds a placeholder such as N/A, or holds anything that is not exactly nine digits, the app has nothing to look for and will never show a position.
To check it, open Vessel Settings → General — reached from the gear button in the top bar, or from the bottom of the menu on a phone. The button only appears if you hold the Vessel Settings → View permission. Look at MMSI under Identity & Registration. Correcting it additionally needs Vessel Settings → Edit: press Edit, put the correct nine-digit number in the MMSI field and press Save.
Once a valid MMSI is in place, the map fills in as soon as the vessel's next broadcast is picked up. Do not expect it within the minute: the list of vessels being listened for is refreshed once a day, so a newly added or corrected MMSI can take until the following day to start producing positions. Nothing is backfilled either — the track begins from the first position received after the number was put right, not from wherever the yacht was before that.
No AIS data has been received
If the MMSI is right, the yacht is simply not being heard at the moment. That is expected for a vessel far offshore, laid up with its transponder switched off, or in an area with no receiver coverage. Nothing needs fixing, and the position will reappear by itself once the vessel is heard again.
Archived vessels are no longer listened for, so an archived yacht's map normally stops updating and keeps showing the last position received before it was archived.
A related case: the chart may show a perfectly good pin whose Last update is old. That is the same situation caught slightly earlier — the app has a position, it is just not a recent one. See How current the position is.