So, I have multiple drawing softwares and a drawing pad and the base art downloaded and everything else I may need. Everything was working. However, I tried painting over the image and nothing happened. I changed brush opacity, I tried different drawing softwares, I tried changing layers but I couldn't paint over the base art no matter what I tried. Help?

Check the image properties - is the image perhaps saved  as read-only? There might be a box you can uncheck that'll allow you to edit it.

Otherwise go Save As - and make sure you don't save it as read only.