You need to buy and replace the DL2032 battery or this system will not boot ever again.  2.2 volts or lower will have that error.

https://www.dell.com/support/home/en-us/product-support/product/optiplex-755/docs

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-755_user%27s%20guide_en-us.pdf

https://www.youtube.com/watch?v=rzzQo_wLcoE

Its unfortunate that the battery can cause this but they are not expensive.

https://www.staples.com/Duracell-DL2032-3-0-Volt-Lithium-Battery/product_273151

Most Grocery stores carry these because blood sugar machines use the same battery.