Gravel bike gears slipping standing up

[me vs my gravel bike]

I currently own a carerra gravel bike. When I try to stand up and pedal the chain and gears slip and this nearly results in me falling off every time.
When I ride the bike I do not stand up and pedal. This means when I’m going up a hill it’s insanely hard to climb it.
What do I do as it’s very hard to ride seated continuously

 

[Ajax Bay]

How many miles have you done since the current chain was fitted? Same Q for cassette.

 

[wafter]

Standing up you're likely putting in more load than sitting, which will exacerbate the effects of wear and promote slipping.

Sounds like the chain and possibly the cassette are worn; the liklihood of which will depend on distance covered, lubing / cleaning regime and conditions... tell us more