full hard drive plenty of iCloud but iMac won't move files says it has no space...

The files that should be moved to iCloud are files you want to share across your Apple devices (iPhone, iPads etc...). iCloud IS NOT a location to that should be used to increase storage space on the internal drive!!!!!

What experienced users do is move large libraries (photos, music, movies) to external HDs. This is safe, simple and smart. Apple explains how to do this for photos in Move your Photos library to save space on your Mac, the process is the same for music and movies. Once you have moved them to an External Hard Disk, simply double click the library and this will launch the associated app and also "teach" the app where the library is now located. Once you have done that, go back to the original library on the internal drive and move it to the Trash and then empty the Trash. The result is more space on the internal drive.

DO NOT buy Walmart quality EHDs, buy high quality which only costs slightly more but provides much longer life due to adequate cooling. A drive that is very trusted by experienced users is the OWC Mercury Elite Pro series. Estimate about how much space you need then select an External Hard Disk with a bit more. For example, if you need 1TB of storage, buy 2TB. Why, to allow for future growth.