Hi,
Based on the error message received, please ask your employee to follow the steps below to fix the issue.
I understand that they are receiving the message “Try another verification method” when attempting to sign in. This message usually appears due to unusual activity, a high volume of requests, or similar factors.
Make sure they are using a device and location they have previously used to sign in. Try a different verification method, such as an alternate email instead of SMS, or vice versa. They can also switch networks by using Wi-Fi or mobile data. Avoid making repeated sign-in attempts. In some cases, waiting 24 to 48 hours may help resolve the issue.
For further assistance, use the sign-in helper tool to identify the problem and explore available support options.
https://go.microsoft.com/fwlink/?linkid=2214157
Reference: See the 'Try another verification method' section: