This is because the "pooluser" which is the Sql user for the Processing Pools connection string in Sitecore is not existing in your database. In order to find this you have to login into the SQL Management Studio and look for the "pooluser" login inside the Security for SqlProcessingPoolsUser. Check the Installationjson files "xconnect-xp0.json" and "sitecore-XP0.json" script you run to install sitecore you can find the "pooluser" there also and the database it has referred tothere. You can create the missing user login manually inside the security to fix this or re-run the installation to fix this.