tl;dr -
If you see the error above, Check your SqlSetting section in the config.json file for DataSource and make sure the charset has utf8mb4 included
Change
"DataSource": "USERNAME:PASSWORD@tcp(127.0.0.1:3306)/bitnami_mattermost?charset=utf8",
For
"DataSource": "USERNAME:PASSWORD@tcp(127.0.0.1:3306)/bitnami_mattermost?charset=utf8mb4,utf8",