can't open my recent score

You don't need to open it to attach it. (In fact, it works better if the file is closed when you attach it.)

Click the Choose a file button, browse to the place you saved the file, and select it.

If you have saved the score locally on your computer, try to locate the MSCZ file in your file manager (which in Mac OS is called Finder) and attach it here.

If you have saved the score to the cloud, please follow the instructions below. In fact, cloud scores are also saved locally on your device, but in a somewhat hidden location. Let me know if something isn't working or if you are using a different operating system than Mac OS.

1. Open Musescore.
2. Go to Diagnostic > System > Show paths in the top menu.
3. Locate userAppDataPath: in the list and double-click on it.
4. Your file manager, Finder, should open. You'll see a folder called cloud_scores. Open it.
5. This folder contains your cloud scores. Try to find the file in question by opening one at a time (and see which one doesn't open) or by looking at when it was last modified.
6. Attach the file here.