Here is the full list of all 46 MXGP 2020 - there are The Official Motocross Videogame achievements.
Reach Prestige level 100
Reach Prestige level 75
Achieve the podium 50 times in Grand Prix
Get a total of 300,000 MX Credits
Cover 45 Km - 31,07 mi in Free Roaming in the Playground
Win a race with physics level set to Advanced
Win a race with AI Difficulty level set to Realistic
Cover 30 km - 18.64 mi in Free Roaming in the Playground
Get a total of 200,000 MX Credits
Reach Prestige level 50
Cover 15 Km - 6,21 mi in Free Roaming in the Playground
Cover a total of 500 km - 310.69 mi
Win a championship in Championship mode
Get a total of 100,000 MX Credits
Win a Grand Prix at Matterley Basin in any game mode
Purchase any bike for the first time
Win a Grand Prix at Uddevalla in any game mode
Win a Grand Prix at Shanghai in any game mode
Win a Grand Prix at Teutschenthal in any game mode
Win a Grand Prix at Loket in any game mode
Reach Prestige level 25
Win a Grand Prix at Imola in any game mode
Win a Grand Prix at Agueda in any game mode
Get 10 Holeshots in any game mode
Win a Grand Prix at Orlyonok in any game mode
Win a Grand Prix at Neuquen in any game mode
Win a race without falling and without using Rewind
Complete a race without using Rewind
Win a Grand Prix at Pietramurata in any game mode
Reach the first position in all sessions in a race set to a Full weekend
Complete a race with race length set to Realistic
Cover a total of 250 km - 155.34 mi
Start replay at the end of a race in any game mode
Access Photo Mode for the first time in any game mode
Save your bike's setup for the first time
Complete a valid lap in Time Attack
Cover a total of 100 Km - 62,14 mi
Win a Grand Prix at Afyonkarahisar in any game mode
Get a Holeshot and finish first in the same race
Make a purchase on the Rider Customisation page for the first time
Win a Grand Prix at Kegums in any game mode
Win a Grand Prix at Valkenswaard in any game mode
Finish first in a race with the weather set to Rain in any game mode
Complete a lap without falling on any track and in any game mode
Perform a Scrub in any game mode
Use Rewind for the first time in any game mode