Mac mini with Mojave 10.14.6 says my memory is running low

My Mac mini 2018 says my memory is running low. I've offloaded almost everything but the storage doesn't increase. The space is maxd out with 'SYSTEM'. How do I reduce the system storage. I'm running Mojave 10.14.6. Im trying to upgrade to Sequoia but haven't got enough disk space to accomadate. Please help

[Re-Titled by Moderator]

Original Title: My macMini 2018 issue