On September 30, 2016, NCBI made significant changes to how their records are accessed. SnapGene version 3.3 and later use the correct format for import requests. Older SnapGene versions will no longer connect to NCBI. To fix this issue you will need to upgrade to the current supported version of SnapGene. As a workaround, you can search on the NCBI website and then export a record as a .gb file that SnapGene can open.