My home was built in 1998 and the supply valve that was put in at the toilets are covered in plastic. The line going from the valve to the toilet is crimped on. The crimp is leaking and I need to find out how to remove the existing valve in order to replace it.

Not sure which supply valve you have, but every valve is removable. You will have to close water for entire house, remove the valve and replace it with new one.

As far as new valve goes - I suggest you get the one that is valve only and doesn't have water supply attached to it. Water supply comes as separate piece. Then, if something like this happens again, you will be able to replace the water supply line without replacing the valve

But before you do anything, can you snap a picture of this valve in question and post it for us so we can ID it for you ?

Back to you

Milo

48269
Here is a picture of the fitting/valve I am in need of replacing as the supply line is crimped on.

That is an old Accor push pull valve, common to mobile homes. You need to turn off the house water, relieve the pressure and then holding the valve, pull back on the part closest to the wall, twist it left and right while pulling back and you should be able to remove it. It is similar to a Gator or Shark bite.The cover doesn't come off, it is the valve. Once off, you can replace with a compression angle stop. You can use a Gator/Shark fitting but removing the old stop tends to scar the copper tubing so make sure to sand off any burrs. See if this link helps.
https://www.bing.com/videos/search?q=accor+valve+replacement&view=detail&mid=917ADCB1762BAC278FBA917ADCB1762BAC278FBA&FORM=VIRE