Are you getting any error messages, when you try to save the song? if yes, what is the message?
Which version of GarageBand are you using? On macOS 10.13 High Sierra you could still be using the older iLife version GarageBand '11, version 6.0.5, or are you using the more recent GarageBand 10?
Have you closed all other apps, that might be hogging the memory?
Where is your project stored? In your GarageBAnd folder or on an external drive?
is it just the "Save" that does not work, or is GarageBand completely unresponsive?
If GarageBand is still responsive, try to save the song from the Share menu (in GarageBand 10). You could save at least each track separately as an audio file try to save the project as an iOS project, GarageBand 10 can can convert the iOS project back to a GarageBand for Mac project.