So on Linux (VM) I have 100GB free storage, however, it is only using 8GB and won’t let me change the amount, please help me!
As in your partition fails to expand? Is this a new partition for Manjaro? Do you have any other partitions on the disk?
Assuming you are using VirtualBox and Manjaro
As I understand your issue, you have resized the virtual disk but your guest system does not see the new space - then you should pay attention to the second step - mentioned in above link - for resizing the virtual disk.
Boot the guest from the Manjaro LIVE ISO, launch Gparted and resize the partition on the virtual disk.