help me
pc specs
amd sempron le -1250
gpu version nvidia geforce 6 series
version 304.137
Do you see some drivers inside here :
I never got so powerful graphical card and have been using only the drivers available inside Software & Updates. If nothing there someone else will for sure help you installing the driver manually. Tell us 
EDIT : I found this link that contains a .run file that suppose to do all the installation :
Hy
Perhaps in this page you will find all the steps for install the driver.
I too have a Geforce 6 (the 6200 - id NV44) but the installer of the driver I have found (NVIDIA-304.125) made all the work until the end when... NOT succeed in rebuilding the kernel.
I'm afraid that the driver is not compatible with the current release of Mate (20.04 Focal Fossa).
Please notify me if you succeed in installing . If not this are the instructions to uninstall the driver.