First check whether the eSIM appears in your phone's SIM settings. An activation error does not always tell you how far installation got.
If the profile is present
Confirm that the line is enabled and that you are in the plan's coverage area. A travel eSIM may wait for an eligible network until you arrive.
At your destination, check the selected mobile-data line, Data Roaming, plan status and validity. See "My eSIM is not working: start here".
Try a brief airplane-mode toggle or restart, then test again. For installation that has not finished, use a stable internet connection and follow the device's on-screen instructions.
If it still fails
Send support the exact error, phone model, iOS version, current country and whether the profile is visible. Check for an iOS update if practical.
Do not delete the eSIM to restart activation. Keeping the profile gives support more recovery options. See "Deleting an eSIM, and when not to".