In 0:30 you are mounting the tyre like a road bike, but this tires fit a lot easier if you put the rim totally inside the tyre, and fit both sides from inner liner (inside of bead) to the outside side of the rim, never outside - outside.
Renthal Twinwall 997 Handlebar 28.6mm Zeta UX3 Clamp Kit 28.6mm Grip Domino A010 Zeta Pivot Set CP Lever Zeta Armor Handguard ZETA Armor XC Protector Handguard Mount Front Type Super zero Front Sping Kit Ohlins FSK159 Tire IRC VX30 80/100-21 + 110/100-18 Rim lock Crankcase and Ignition/Clutch covers Skid plate Yamoto Aluminium Brake Pedal Super Zero Foot Pegs DRC Shift lever Super zero Exhaust Yoshimura Radiator Guard Super Zero Rear shock Ohlins HO129 Rising Links Kit Astront 1.5" Side Stand CRF250Rally 2017 Tail Tidy Yamoto Chain Jomthai 520 Black/Gold Rear Sprocket Jomthai Aluminium 45T Front Sprocket Jomthai 13 T Chain Guide Zeta Swingarm Protectors