sprint animation stop playing when i restart walking ,how can i fix?

you need to play the animation whenever the character starts running, not just when the player presses shift

humanoid.Running:Connect(function(speed)
    if speed > 0 then
		if isSprinting and not sprintAnim.IsPlaying then
			sprintAnim:Play()
		end
        sprintAnim:AdjustSpeed(speed/26) -- pretty sure you'd want the animation's playback speed to change based on how fast the player is actually walking 
    else
        sprintAnim:Stop()
    end
end)

also, the player could press shift when they're standing still, so you'd need to check for that to make sure the sprinting animation doesn't play while they're not moving

UserInputService.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        if not isSprinting then
            isSprinting = true
			if humanoid.MoveDirection.Magnitude > 0 then -- just checking the player's MoveDirection is the easiest way to tell if they're not pressing any movement keys
				sprintAnim:Play()
				sprintAnim:AdjustSpeed(0)
			end
            humanoid.WalkSpeed = 26
            humanoid.JumpPower = 50
        else
            sprintAnim:Stop()
            humanoid.WalkSpeed = 16
            humanoid.JumpPower = 50
            isSprinting = false 
        end 
    end
end)