I’m not sure if this is a XO question or problem – more of a partition problem. When you installed Debian I’m betting the installer probably allocated you a very small /opt partition – just a hunch. When trying to utilize this partition you’re basically using the entire space. I guess this is all falls back to how you setup your disc partitions during installation – either manually or automatically. You might want to revisit your disc partitioning scheme(s). Usually for me with my VMs if I’m not using one big fat / partition, I’m using a separate /boot partition, / = system partition, and possibly a /home partition. The separate /boot partition is more for UEFI since whatever partition it boots from needs to be formatted FAT32 and the the other partitions are what every scheme you like – ie ext4, ext3, etc.