service is part of the URL and therefore is required.

.changelog/3720.txt

@@ -0,0 +1,4 @@
+```release-note:breaking-change
+`app_engine_flexible_app_version` and `app_engine_standard_app_version` now require `service` to be set.  Before this change, leaving the value empty would have passed validation but failed during apply.
+
+```

google/resource_app_engine_flexible_app_version.go

@@ -149,6 +149,12 @@ replies to a healthcheck until it is ready to serve traffic. Default: "300s"`,
 				Required:    true,
 				Description: `Desired runtime. Example python27.`,
 			},
+			"service": {
+				Type:             schema.TypeString,
+				Required:         true,
+				DiffSuppressFunc: compareSelfLinkOrResourceName,
+				Description:      `AppEngine service resource`,
+			},
 			"api_config": {
 				Type:        schema.TypeList,
 				Optional:    true,
@@ -794,12 +800,6 @@ Please see the app.yaml reference for valid values at https://cloud.google.com/a
 				Optional:    true,
 				Description: `The path or name of the app's main executable.`,
 			},
-			"service": {
-				Type:             schema.TypeString,
-				Optional:         true,
-				DiffSuppressFunc: compareSelfLinkOrResourceName,
-				Description:      `AppEngine service resource`,
-			},
 			"serving_status": {
 				Type:         schema.TypeString,
 				Optional:     true,

google/resource_app_engine_standard_app_version.go

@@ -104,6 +104,12 @@ All files must be readable using the credentials supplied with this call.`,
 				Required:    true,
 				Description: `Desired runtime. Example python27.`,
 			},
+			"service": {
+				Type:             schema.TypeString,
+				Required:         true,
+				DiffSuppressFunc: compareSelfLinkOrResourceName,
+				Description:      `AppEngine service resource`,
+			},
 			"automatic_scaling": {
 				Type:        schema.TypeList,
 				Optional:    true,
@@ -388,12 +394,6 @@ Modules API set_num_instances() you must use 'lifecycle.ignore_changes = ["manua
 				Description: `The version of the API in the given runtime environment.
 Please see the app.yaml reference for valid values at https://cloud.google.com/appengine/docs/standard//config/appref`,
 			},
-			"service": {
-				Type:             schema.TypeString,
-				Optional:         true,
-				DiffSuppressFunc: compareSelfLinkOrResourceName,
-				Description:      `AppEngine service resource`,
-			},
 			"threadsafe": {
 				Type:        schema.TypeBool,
 				Optional:    true,

website/docs/r/app_engine_flexible_app_version.html.markdown

@@ -150,6 +150,10 @@ The following arguments are supported:
   (Required)
   Health checking configuration for VM instances. Unhealthy instances are killed and replaced with new instances.  Structure is documented below.
 
+* `service` -
+  (Required)
+  AppEngine service resource
+
 
 The `readiness_check` block supports:
 
@@ -311,10 +315,6 @@ The `liveness_check` block supports:
   (Optional)
   A service with manual scaling runs continuously, allowing you to perform complex initialization and rely on the state of its memory over time.  Structure is documented below.
 
-* `service` -
-  (Optional)
-  AppEngine service resource
-
 * `project` - (Optional) The ID of the project in which the resource belongs.
     If it is not provided, the provider project is used.
 

website/docs/r/app_engine_standard_app_version.html.markdown

@@ -129,6 +129,10 @@ The following arguments are supported:
   (Required)
   Code and application artifacts that make up this version.  Structure is documented below.
 
+* `service` -
+  (Required)
+  AppEngine service resource
+
 
 The `deployment` block supports:
 
@@ -220,10 +224,6 @@ The `files` block supports:
   (Optional)
   A service with manual scaling runs continuously, allowing you to perform complex initialization and rely on the state of its memory over time.  Structure is documented below.
 
-* `service` -
-  (Optional)
-  AppEngine service resource
-
 * `project` - (Optional) The ID of the project in which the resource belongs.
     If it is not provided, the provider project is used.