Its your skins str file.
make sure the str file has 2 shop cars listed like the following - I beleve i had the same issue a few years back.
[shop]
num cars = 2
[shop car 0]
cost = 0
avail = qm
front left toe = 0.000000
front right toe = 0.000000
rear left toe = 0.000000
rear right toe = 0.000000
front left offset = 0.000000
front right offset = 0.000000
rear left offset = 0.000000
rear right offset = 2.000000
compound available = {1,1,1,1,1}
front left condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
front right condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
rear left condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
rear right condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
tire width level = 2
bleedable tires = 0
engine level = 1
engine condition = 1.000000
performance level = 2
exhaust level = 2
carburetor level = 2
gearbox level = 2
gearbox condition = 1.000000
chassis condition = 1.000000
spring strengths = {100.000000,100.000000,100.000000,100.000000}
bump dampenings = {80.000000,80.000000,80.000000,80.000000}
rebound dampenings = {80.000000,80.000000,80.000000,80.000000}
tire pressures = {12.000000,12.000000,12.000000,12.000000}
rear spoiler = 0.000000
brakes level = 2
suspension level = 2
time multiplier mean = 0.870452
time multiplier std dev = 0.266741
[shop car 1]
cost = 0
avail = r
front left toe = 0.000000
front right toe = 0.000000
rear left toe = 0.000000
rear right toe = 0.000000
front left offset = 0.000000
front right offset = 0.000000
rear left offset = 0.000000
rear right offset = 2.000000
compound available = {1,1,1,1,1}
front left condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
front right condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
rear left condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
rear right condition = {1.000000,1.000000,1.000000,1.000000,1.000000}
tire width level = 2
bleedable tires = 0
engine level = 1
engine condition = 1.000000
performance level = 2
exhaust level = 2
carburetor level = 2
gearbox level = 2
gearbox condition = 1.000000
chassis condition = 1.000000
spring strengths = {100.000000,100.000000,100.000000,100.000000}
bump dampenings = {80.000000,80.000000,80.000000,80.000000}
rebound dampenings = {80.000000,80.000000,80.000000,80.000000}
tire pressures = {12.000000,12.000000,12.000000,12.000000}
rear spoiler = 0.000000
brakes level = 2
suspension level = 2
time multiplier mean = 0.870452
time multiplier std dev = 0.266741