No database selected checking category id

https://www.samanthahardy.com.au/

Help! This blog worked just fine on the development server, but when uploaded to its final destination I receive this message: “No database selected checking category id”. I have deleted the database and recreated it, also re-uploaded all of the wordpress files from the server which works (these are located at https://www.mariagrist.net/wordpress) but still receive the above error. Is this because the server is using SFTP? or have I done something wrong? My knowledge of php and databases is practically non-existent so I am stuck on troubleshooting this one. All help gratefully received, thanks!!!