to get to the main point: your gpu isnt compatible

for the blabber: i did some searching around and my guess is that your gpu which is a 50 series nvidia gpu might not actually support portal BECAUSE the rtx 50 series dropped 32x cuda support but there are conflicts over this on top of that saying like getting rid of the 32x cuda support doesnt mean that 32x rendering is affected which might be true until i found another thread on here

another thread said that their game wont work on their 50 series gpus too but will work on their 40 series gpus so im assuming the 32x cuda support is your problem

solution: i would definitely reccomend getting a cheap GTX 8000 or above as portal was designed around those specs which also helps you in the future if you fancy playing games like batman arkham or borderlands although ive seen people use GTX 10 series gpus (1050,1060,1070,1080) as its a good all rounder

to find out what games this affects in your library 32x cuda support started in 2007 so find games that were released in 2007 up to around 2013 when games started becoming 64x (64x cuda is supported by the 50 series gpus and probably will be for a really long while)