Yes- coolant reservoir is designed to accept excess coolant - As coolant heats up it will expand and flow to the coolant recovery tank (reservoir) and as it cools (contracts) it will be sucked back into the radiator
Engine coolant expands when heated. Heated coolant needs a reservoir to come and go as needed.
Fill the reservoir with G12 VW coolant only and it will go into the engine.
Either the coolant reservoir is low on coolant or the coolant level sensor is NG.
The coolant expands when heated, contracts when cool. The coolant reservoir gives the coolant a place to come and go as needed.
It probably has a leak. Here's something to check - the radiator has a tube coming out to the reservoir, so when it gets hot and expands the coolant goes into the reservoir and should go back into the radiator when it cools, but if your reservoir has a crack and the coolant leaks out, well it's gone and can't go back into the radiator - this would cause a lot of loss of coolant.
Most vehicles have a coolant reservoir (a clear plastic tub) with a phrase "engine coolant only" printed on the cap. This is where you should add engine coolant.
Air bubbles remaining in the system
In a 1999 VW Cabrio, antifreeze is added to the coolant reservoir, which is typically located near the engine in the front of the vehicle. The reservoir is usually a translucent plastic tank with a cap labeled for coolant. Make sure to check the coolant level and add the appropriate type of antifreeze mixed with water, following the manufacturer's specifications. Always allow the engine to cool before opening the reservoir to prevent injury.
You need to get one. Cars need it in the case if it's too much of the coolant. Without the reservoir the coolant in the case when there is some extra will go on the ground. Which is in some states is a crime because it's hazardous and not nature friendly.
Probably low on coolant, check the reservoir.
Hoses from the coolant reservoir cap typically connect to the radiator and the engine cooling system. They allow coolant to flow between the reservoir and the radiator, helping to maintain proper coolant levels and pressure in the system. This ensures efficient heat transfer and prevents the engine from overheating. Some systems may also have hoses that direct excess coolant back to the reservoir when the engine is hot.
A surge tank is basically a pressurized coolant reservoir. As your engine heats and cools the coolant expands and contracts. Because of this You need a place for extra coolant to go to when it expands and be drawn from when it contracts. Conventionally the recovery tank or reservoir was not pressurized. A tube from the radiator cap allowed coolant to be sucked and drained in to the reservoir. However, on many newer vehicles like yours the reservoir is pressurized and called a surge tank.