When an employee is unable to access the company portal using a VPN on their personal smartphone, the most likely cause is:
The smartphone is not enrolled in MDM service: Mobile Device Management (MDM) services often control access to company resources. If the smartphone is not enrolled, it may be blocked from accessing the VPN and, consequently, the company portal.
The application must be purchased by the company: Unlikely, as most VPN applications are free to download or included in the company's software package.
The smartphone is not on the latest OS version: While this could cause compatibility issues, it is less likely than the MDM enrollment issue.
The application fails to install and launch: This would be a different problem and would usually present specific error messages.
Reference:
CompTIA A+ 220-1102 Exam Objectives, Section 2.7: Explain common methods for securing mobile and embedded devices.
MDM configuration and access control documentation.