2 problems...believe they are related.

Logging in on a specific user instantly logs me back out.
1) can log in as root
2) tried changing desktops (tasksel) but ran into "apt-get failed (100)"
3) checked sources.list and it looked good
4) ran "apt-get update" and saw Error writing to output file - write (28: No space left on device)"
5) ran df -h & saw 100% usage on sda1

I am assuming there is an error with the user profile because of lack of space. If I remember right when I installed Debian 10 in the config I assigned 11gb. It is running in a virtualbox which I assigned 20GB (I assumed the debian partition would just grow (my bad) Also in the VM I have set it to dynamically assign storage.

Question
1) How do I expand the size of the partition?
2) Does dynamically allocated storage in Virtual box work with linux? I thought the point was that if you assigned 20G but then started adding files to say 30GB virtual box would grow that partition to 30GB. Seems like I would have to manually resize the partition
3) If I manually set the partition to say 30GB will it auto expand the virtual box dynamically allocated storage to 30GB.

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Correct, ext4 filesystems have reserved space which allows root to login in case the filesystem becomes full but a regular user can not.

1. Using VirtualBox media manager you can increase the size of the virtual disk. Then using linux tools you can expand your partitions. While you can expand ext4 partitions on the fly you will also need to probably either move or delete your swap partition and it might be easier using a live CD.

2. Yes, dynamic storage works. My debian 10 VM VDI is around 16 GB but I am only using 11 GB of actual storage space. 9 for / partition and 2 for swap. The output of df does not include swap. As previously posted you will need to manually expand the partitions within linux to include the added virtual space. The guest sees the drive at its full capacity but the host vdi file only shows what is actually used.

3. Eventually

You should check for some runaway process filling your filesystem. My usual first suspect is something logging to /var/log or writing dump files to /var/crash.

The size of the block device may change, but the filesystem was created with a set size. The command to resize it depends on the type of filesystem. You can check with:

In addition, posting the output of the lsblk will show us how the drive is partitioned.