Where would I find the error log sorry?

I installed application insights, these are the exceptions:

Microsoft.Azure.WebJobs.Host.FunctionInvocationException: Exception while executing function: UpdateSiteBinding
---> Microsoft.Azure.Management.WebSites.Models.DefaultErrorResponseException: Operation returned an invalid status code 'Forbidden'
at Microsoft.Azure.Management.WebSites.WebAppsOperations.BeginCreateOrUpdateWithHttpMessagesAsync(String resourceGroupName, String name, Site siteEnvelope, Dictionary2 customHeaders, CancellationToken cancellationToken) at Microsoft.Azure.Management.WebSites.WebAppsOperations.CreateOrUpdateWithHttpMessagesAsync(String resourceGroupName, String name, Site siteEnvelope, Dictionary2 customHeaders, CancellationToken cancellationToken)
at Microsoft.Azure.Management.WebSites.WebAppsOperationsExtensions.CreateOrUpdateAsync(IWebAppsOperations operations, String resourceGroupName, String name, Site siteEnvelope, CancellationToken cancellationToken)
at Microsoft.Azure.WebJobs.Host.Executors.VoidTaskMethodInvoker2.InvokeAsync(TReflected instance, Object[] arguments) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\VoidTaskMethodInvoker.cs:line 20 at Microsoft.Azure.WebJobs.Host.Executors.FunctionInvoker2.InvokeAsync(Object instance, Object[] arguments) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionInvoker.cs:line 52
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.InvokeWithTimeoutAsync(IFunctionInvoker invoker, ParameterHelper parameterHelper, CancellationTokenSource timeoutTokenSource, CancellationTokenSource functionCancellationTokenSource, Boolean throwOnTimeout, TimeSpan timerInterval, IFunctionInstance instance) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 572
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithWatchersAsync(IFunctionInstanceEx instance, ParameterHelper parameterHelper, ILogger logger, CancellationTokenSource functionCancellationTokenSource) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 518
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithLoggingAsync(IFunctionInstanceEx instance, FunctionStartedMessage message, FunctionInstanceLogEntry instanceLogEntry, ParameterHelper parameterHelper, ILogger logger, CancellationToken cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 296
--- End of inner exception stack trace ---
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithLoggingAsync(IFunctionInstanceEx instance, FunctionStartedMessage message, FunctionInstanceLogEntry instanceLogEntry, ParameterHelper parameterHelper, ILogger logger, CancellationToken cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 343
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.TryExecuteAsync(IFunctionInstance functionInstance, CancellationToken cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 105

Just incase, I've verified that the function app has "web plan contributor" and "website contributor" roles on the target resource group

It looks like a 403 error in Azure REST API, so it seems to be a problem around permissions. There may be more detailed error messages left in the Activity Log of the resource group.
https://docs.microsoft.com/en-us/azure/azure-monitor/essentials/activity-log