Elasticsearch index file needs to be re-index in old version

[MrGeneration]

@FunkStar

#<UserAgent::Result:0x00005592c7788048 @success=false, @body=“{"error":{"root_cause":[{"type":"parse_exception","reason":"No processor type exists with name [attachment]","header":{"processor_type":"foreach"}}],"type":"parse_exception","reason":"No processor type exists with name [attachment]","header":{"processor_type":"foreach"}},"status":400}”, @data=nil, @code=“400”, @content_type=nil, @error=“Client Error: #<Net::HTTPBadRequest 400 Bad Request readbody=true>!”>

You tried to install mapper-attachments which is a attachment plugin pre elasticsearch 5.6 (According to your console output, you installed 5.6.16). This should have thrown an error - anway.
Do the following to solve this:

sudo /usr/share/elasticsearch/bin/elasticsearch-plugin remove ingest-attachment #safety IF any was installed
sudo /usr/share/elasticsearch/bin/elasticsearch-plugin install ingest-attachment
systemctl restart elasticsearch
zammad run rake searchindex:rebuild