QB Issue Resolution:

120G must be hard drive space and not memory since memory maxes out for most systems at 12G

right click on the computer icon on the desktop or in windows explorer and select properties the window will show installed memory (RAM)

Resolution for Issue 'install error stating “only 3mb free” but have over 20GB free' available: Yes (Solved).
Source: Intuit Community forum.

install error stating “only 3mb free” but have over 20GB free: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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