While using Liferay Sync, a user may experience the following error when attempting to connect to an instance of the Liferay Portal:
The portal version could not be verified. The minimum supported EE version is 6.1.20
If your portal is at or beyond the requirements specified in the warning, then the issue likely lies in the header.
Liferay currently uses the portal header to detect the portal version. Therefore, if the header is set to minimal or none in the portal-ext.properties, Liferay will not detect the portal version (and the above warning will be thrown).
To resolve this issue, simply make sure the header contains the version of the portal that you are using. This can be done with the following property: