JCOBridge will not support officially .NET 5 until its official release declared on November 2020 (see https://devblogs.microsoft.com/dotnet/introducing-net-5/).
A workaround to support .NET 5 on JCOBridge is the following one:

1. browse the folder packages\jcobridge\2.2.1\build and open the file JCOBridge.targets
2. replace its whole content with the following:

   <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
       <ItemGroup>
           <net461 Include="$(MSBuildThisFileDirectory)\..\contentFiles\net461\*.*" />
       </ItemGroup>
       <ItemGroup>
           <netcoreapp31 Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\*.*" />
           <netcoreapp31winx86 Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\win-x86\*.*" />
           <netcoreapp31winx64 Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\win-x64\*.*" />
           <netcoreapp31winarm Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\win-arm\*.*" />
           <netcoreapp31winarm64 Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\win-arm64\*.*" />
           <netcoreapp31linuxx64 Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\linux-x64\*.*" />
           <netcoreapp31linuxarm Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\linux-arm\*.*" />
           <netcoreapp31linuxarm64 Include="$(MSBuildThisFileDirectory)\..\contentFiles\netcoreapp3.1\linux-arm64\*.*" />
       </ItemGroup>
   
       <Target Condition="$(TargetFramework.Contains('net4'))" Name="CopyFiles461" BeforeTargets="Build"> 
           <Copy SourceFiles="@(net461)" DestinationFolder="$(TargetDir)" />
       </Target>
       <Target Condition="$(TargetFramework.Contains('netcoreapp')) Or $(TargetFramework.Contains('net5'))" Name="CopyFilesCore" BeforeTargets="Build">
           <Copy SourceFiles="@(netcoreapp31)" DestinationFolder="$(TargetDir)" />
           <Copy SourceFiles="@(netcoreapp31winx86)" DestinationFolder="$(TargetDir)/win-x86" />
           <Copy SourceFiles="@(netcoreapp31winx64)" DestinationFolder="$(TargetDir)/win-x64" />
           <Copy SourceFiles="@(netcoreapp31winarm)" DestinationFolder="$(TargetDir)/win-arm" />
           <Copy SourceFiles="@(netcoreapp31winarm64)" DestinationFolder="$(TargetDir)/win-arm64" />
           <Copy SourceFiles="@(netcoreapp31linuxx64)" DestinationFolder="$(TargetDir)/linux-x64" />
           <Copy SourceFiles="@(netcoreapp31linuxarm)" DestinationFolder="$(TargetDir)/linux-arm" />
           <Copy SourceFiles="@(netcoreapp31linuxarm64)" DestinationFolder="$(TargetDir)/linux-arm64" />
       </Target>
   </Project>
   	

3. rebuild your project which targets .NET 5 (TFM: net5.0)