Not sure what you have... what I’m familiar with is 3/8”compression by 7/8” for a toilet tank supply. It’s possible your supply valve is 1/2” compression. There’s adapters to go 1/2-3/8” compression. Unless you do have 1/2” pipe threads there’s a supply line like this. https://www.homedepot.com/p/Fluidma...nector-with-3-8-in-Adaptor-B4T12CSW/206050471