I have free space, but I can still not download what I want

I found this old thread from two years ago which has the same issue with you. Refer to this link: https://discussions.apple.com/thread/8001168

I haven't tried re-indexing spotlight before but you could try it since the solution there was 'Apple Recommended' which means it was approved by Apple.

You're downloading a file that will take up half of you're available space. Apple has always stated that you need at least 25% of your hard drive or SSD available for efficient operation. The system will prevent you from installing files if you don't abide by that rule. The system needs room to access the drive for virtual memory when paging out and to run system operations. The storage drive is not just to store files, that's why 25% of your total drive space needs to be available.

Free up more space (your storage is full), reboot through Safe Mode, and try again.