Hi @Mary_Cane_Bandohan Welcome the community
Actually if you look closely.. the comparability reads.
^ Elasticsearch output compatibility - Beats, Logstash, and Elastic Agent indexing data to Elasticsearch
Meaning 7.14 beats are compatible as an output destination (i.e filebeat can write to Elasticsearch), it does not say that all functionality is compatible... in this This case as @legoguy1000 points out the ngnix module has evolved and is using new fields / capabilities that did not exist in 7.8.1.
You should perhaps consider upgrading elasticsearch.
Also in general I would consider it best practices to keep elasticsearch "ahead" of beats as recommended in the upgrade guide here