OpenGL effect build error .NET5

I found a neat tcp network lib I wanted to try out.
Th lib is in .NET5 so I used monogame opengl which can run using .NET5
(but I really want to use DirectX with .NET5 tho).

I tried to build this effect

#if OPENGL
#define SV_POSITION POSITION
#define VS_SHADERMODEL vs_3_0
#define PS_SHADERMODEL ps_3_0
#else
#define VS_SHADERMODEL vs_4_0_level_9_1
#define PS_SHADERMODEL ps_4_0_level_9_1
#endif

matrix WorldViewProjection;
matrix World;
matrix View;
matrix Projection;
float4 Color = float4(1, 1, 1, 1);
float2 Tiling = float2(1, 1);
float2 Offset = float2(0, 0);
bool useTexture = true;
bool FlipX = false;
bool FlipY = false;

Texture2D Texture : register(t0);
sampler TextureSampler : register(s0)
{
	Texture = (Texture);
	MinFilter = Point; // Minification Filter
	MagFilter = Point; // Magnification Filter
	MipFilter = Point; // Mip-mapping
	AddressU = Wrap; // Address Mode for U Coordinates
	AddressV = Wrap; // Address Mode for V Coordinates
};

struct VertexShaderInput
{
	float4 Position : POSITION0;
	float2 TextureUVs : TEXCOORD0;
	float3 Normal : NORMAL0;
};

struct VertexShaderOutput
{
	float4 Position : SV_POSITION;
	float2 TextureUVs : TEXCOORD0;
	float3 Normal : NORMAL0;
};

VertexShaderOutput MainVS(in VertexShaderInput input)
{
	VertexShaderOutput output = (VertexShaderOutput)0;

	output.Position = mul(input.Position, WorldViewProjection);
	output.Normal = mul(input.Normal, World);
	output.TextureUVs = input.TextureUVs;

	return output;
}

float4 MainPS(VertexShaderOutput input) : COLOR
{
	if (useTexture) {

		float U = input.TextureUVs.x + Offset.x;
		float V = input.TextureUVs.y + Offset.y;

		if (FlipX && !FlipY) {
			return tex2D(TextureSampler, float2(1 - U, V)* Tiling) * Color;
		}
		if (FlipY && !FlipX) {
			return tex2D(TextureSampler, float2(U, 1 - V) * Tiling) * Color;
		}
		if (FlipX && FlipY) {
			return tex2D(TextureSampler, float2(1 - U, 1 - V)  * Tiling) * Color;
		}
		return tex2D(TextureSampler, float2(U, V)  * Tiling) * Color;
	}
	else {
		return Color;
	}
}

technique BasicColorDrawing
{
	pass P0
	{
		VertexShader = compile VS_SHADERMODEL MainVS();
		CullMode = NONE;
		PixelShader = compile PS_SHADERMODEL MainPS();
	}
};

Sadly I got an error using opengl

MonoGame.Content.Builder.Task.targets(138, 5): [MSB3073] Der Befehl "dotnet C:\Users\asame\.nuget\packages\monogame.content.builder.task\3.8.0.1641\build\\..\tools\netcoreapp3.1\any\mgcb.dll /quiet /@:"C:\Users\asame\RiderProjects\Monogame_Online_RPG\Monogame_Online_RPG\Content\Content.mgcb" /platform:DesktopGL /outputDir:"C:/Users/asame/RiderProjects/Monogame_Online_RPG/Monogame_Online_RPG/Content/bin/DesktopGL/Content" /intermediateDir:"C:/Users/asame/RiderProjects/Monogame_Online_RPG/Monogame_Online_RPG/Content/obj/DesktopGL/Content" /workingDir:"C:/Users/asame/RiderProjects/Monogame_Online_RPG/Monogame_Online_RPG/Content/"" wurde mit dem Code 2 beendet.

I created a new project using VS2019 and now I am able to work with DirectX and .NET5…
I opened the project with Rider and everything is working