how to fixlow memory

Please refer to: Activity Monitor to read System Memory and determine how much RAM is being used after reading it you can then determine what is using most of your RAM. If you have an iMac that can have the RAM upgraded, it would be wise to upgrade it.

Are you running any third-party "cleaning" apps? Some may throw out a spurious low memory warning.

Please tell us your iMac sub-model of your iMac and confirm the macOS version. Also tell us how much physical RAM is installed. Both are available by doing "About this Mac..." from your Apple menu.

Can you tell if that warning only shows up from one specific app?