Following the advice from Mickael, I have updated a package in Nuget.
Uninstalled the old Microsoft.Net.Compilers package and installed a replacement package called Microsoft.Net.Compilers.Toolset. Like the question below, we were using the old 1.0.0.0 compilers package which caused the issue.
[C# Files are not being analyzed with Sonar Cloud and Azure DevOps pipeline]