Hi there, you have mentioned that you used this SanDisk Extreme Portable SSD on Windows all along, so I can tell that this drive is Windows NTFS-formatted. If you want to access this drive on macOS devices, a macOS-supported file system is a must. You have to change this drive to a FAT file system or APFS file system. Note that the action to format this SSD to another file system erases everything on it, so you'd better make a full backup beforehand.
Or, if you want to access an NTFS-formatted drive on a Mac without data loss, iBoysoft NTFS for Mac is suggested, which gives you full read/write access to an NTFS drive on a Mac. Once you download this software, it mounts your NTFS drive in read-and-write mode automatically.