I tried to access a REST API through GET method and it worked fine for me. The error could have occurred when the Virtual DataPort Server could not access the REST API.
When accessing a REST API, I would ensure that proper authentication method is chosen so that the Virtual DataPort server could recognize API and connect to the server.
For more information on the error, you could check the VDP.log file located in <Denodo-Home>/logs/vdp.
Refer the section [HTTP](https://community.denodo.com/docs/html/browse/6.0/vdp/administration/creating_views/importing_data_sources_and_creating_base_views/path_types_in_virtual_dataport#http) of the Virtual DataPort Administration Guide for more information on supported authentication methods in Denodo Platform.
Hope this helps you!