@Darren follow these steps to get acquire the token you need in auth header. Also check for any typos in the subscription id that you're using in your Uri.
# Log in first with Connect-AzAccount if not using Cloud Shell
$azContext = Get-AzContext
$azProfile = [Microsoft.Azure.Commands.Common.Authentication.Abstractions.AzureRmProfileProvider]::Instance.Profile
$profileClient = New-Object -TypeName Microsoft.Azure.Commands.ResourceManager.Common.RMProfileClient -ArgumentList ($azProfile)
$token = $profileClient.AcquireAccessToken($azContext.Subscription.TenantId)
$authHeader = @{
'Content-Type'='application/json'
'Authorization'='Bearer ' + $token.AccessToken
}
# Invoke the REST API
$restUri = 'https://management.azure.com/subscriptions/{subscriptionId}?api-version=2020-01-01'
$response = Invoke-RestMethod -Uri $restUri -Method Get -Headers $authHeader