You are confusing “databases” with tables.
A database in MySQL holds tables which store data. Notice that you have a database called “Nextcloud.” This will have your tables that hold all your Nextcloud data, such as users, contacts, and blocked IP’s. Think of MySQL like a folder of Excel spreadsheets if you will. Each “database” is a file in the folder, and each “table” is like a separate workbook/table within that file. You have a file called Nextcloud and inside that file is all the info relate to to Nextcloud.
So, you need to open up the Nextcloud database first, using
USE Nextcloud;
Next, to show all tables, use:
SHOW TABLES;
To show all values from the oc_bruteforce_attempts table, use:
SELECT * FROM oc_bruteforce_attempts;
To remove delete an IP from the table, use:
DELETE FROM oc_bruteforce_attempts WHERE IP="xxx.xxx.xxx.xxx";