Cant download Big Sur on my Mac Book Air says I dont have enough memory but I do ????

You need at least 35.5 GB and as much as 44.5 GB free space to download and install Big Sur: How to upgrade to macOS Big Sur - Apple Support

kenaicameron wrote:

says I don't have enough memory but I do ?

No you do not. The message does not lie.

You require plenty of extra space in addition to the space occupied by the downloaded file. The installation process requires working space.

There are workarounds if you’re interested.