I purchased a vehicle in 2018 and only use it for work (Uber) and I'm wondering if I can use accelerated depreciation and claim the whole cost?

No, you cannot expense a vehicle specifically purchased for Uber.  A vehicle used for hire (Uber is considered a taxi used for hire) is specifically excluded from the Section 179 expense rule.

Business vehicles are cars, SUVs and pickup trucks that are used for business activities.

What does not qualify:

• Vehicles used as equipment, such as dump trucks
• Vehicles used for hire, such as taxi cabs or airport transport vans

https://turbotax.intuit.com/tax-tips/small-business-taxes/business-use-of-vehicles/L6hi0zzzh