Hi.
I will try to help you. Sometimes that happens in SC.
Try logging out of SC and then logging back in with your account.
see if in the configuration of your SC account you have active permissions to connect with Music Studio.
If everything is correct and you still cannot upload music, you can do it in another way:
In the MS audio folder you will have your file converted into wav format, even if you have not been able to send it to SC.
You can export it to apple files or icloud, wherever you want.
go to SC and in the menu of your profile look for “tracks”. Select it. from there, in the upper left corner you have the upload option, select it: it will open files and select what you want to upload.
This last option never fails.
Cheers.