Since Let's Encrypt changed their API from v01 to v02 it isn't possible to request certificates when using DSM 6.1.X.   Workaround: exchange the syno-letsencrypt module with a version from a DSM 6.2.X release.   Preparations:   enable SSH access on your box download this file (from an original DS920+, DSM 6.2.3-25426 Update 3), extract it and copy it to a shared folder of your NAS: https://mega.nz/file/889yCJ5S#Y3UtxPe_qQfRkpxQeALzsFEAIx_NDxLVx2FPLu9HT_I