What does a flashing file folder mean? My computer has not been used in over 2 years And has not been plugged /powered. I have MacBook Pro 2017. TIA

See information in If your Mac starts up to a question mark - Apple Support

While booted to the macOS installer (Internet Recovery Mode or USB installer) you will need to launch Disk Utility to see whether the physical SSD appears on the left pane of Disk Utility. Within Disk Utility you may need to click "View" and select "Show All Devices" before the physical drive appears on the left pane of Disk Utility.

There is a possibility the data on the SSD has become corrupted if the SSD has not received any power to refresh the SSD's NAND memory cells in two years. You may need to erase the whole physical SSD before reinstalling macOS and restoring from a backup. FYI, you should also make sure to keep the battery properly charged when the laptop is being stored (keep the battery with a charge between 20% to 50% since the battery can become permanently damaged if the battery is left completely discharged for an extended time).

If you have a non-touchbar model, then the SSD may actually have failed if the physical SSD does not show up in Disk Utility since the SSD in that particular model has a high rate of failure.