Can't Upgrade car and Race

[ManteoMax]

From the FAQ pinned at the top of the forum:
because career event PI restrictions are not always equal to the class maximum.

@ManteoMax

FM6 FAQ: Stories of Motorsport

Forums: Events List thread - with list of eligible and non-eligible cars for each Volume
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Restrictions for each series are not shown but you can determine them by opening one, selecting Buy Cars, and seeing where each car PI is, or buy entering with a low PI stock car which will prompt you to upgrade, and selecting Manual Upgrade will show the PI limit. If you see an “Illegal Car” message you will have to remove upgrades on your car to reduce the PI to make it legal. Buying cars from this menu includes automatically selected upgrade parts to bring the car to the top of the restriction limit; alternately you can back out of the event and go to the main menu Buy Cars list, buy a car stock, and then upgrade it to your preference. Note that some series have a PI restriction lower than the maximum for that class, so some downloaded tuning setups won’t be eligible. Opponent Drivatars in Stories of Motorsport are upgraded to the top of the event restriction and do not adjust down based on the player’s chosen car, so using a stock car will be less competitive.

Class maximum PI thresholds: E300 | D400 | C500 | B600 | A700 | S800 | R900 | P998 | X999

Stories of Motorsport Volumes and Series restrictions

Click links or scroll down to see eligible cars for each Volume.

Volume 1 - Super Street

• Japanese Street Kings, class: C500
• Compact Sport Coupe, class: D400
• Hot Hatchbacks, class: D400
• Ultimate SpoCom, class: B600
• Rally Day Heroes, class: C500
• Classic Compact, class: E300
  Volume 2 - Sports Icons
• Ultimate Sport Machines, class: A660
• Modern Sport Legends, class: B600
• Executive Heavyweights, class: A650
• Track Toys, class: A700
• Sport Utility Juggernauts, class: C500
• Historic American Muscle, class: D400
  Volume 3 - Grand Touring
• Modern Hypercar, class: R880
• Sport GT, class: S750
• Exotic GT, class: S800
• Birth of the Supercar, class: S800
• Ultimate Grand Tourer, class: S750
• Vintage Touring, class: D400
  Volume 4 - Professional Racing
• Modern GT Racing, class: R865
• V8 Supercars, class: S800
• Early GT Racing, class: R850
• Global Touring Cars, class: S800
• Formula E, class: S750
• Historic Sportscar Racing, class: A675
  Volume 5 - Ultimate Motorsport
• Endurance Prototypes, class: P998
• Legends of Grand Prix, class: S775
• Can-Am, class: R855
• IndyCar, class: X999
• 1970s Grand Prix, class: R900
• Modern Grand Prix, class: X999