FAQ - ANSEL encoding
Error: converting GEDCOM files from ANSEL encoding to UTF-8 encoding not currently supported.
webtrees does not currently have the ability to import GEDCOM files with ANSEL encoding.
(Although code has been written to do this, it requires PHP 7.0. It cannot be included with webtrees, which needs to run on earlier versions of PHP.)
You can use an online conversion tool at dev.webtrees.net/ansel.