That should be the whole thing

build.gradle

@@ -7,6 +7,9 @@ version = '1.0-SNAPSHOT'
 
 repositories {
     mavenCentral()
+    maven {
+        url = "https://repo.papermc.io/repository/maven-public/"
+    }
     maven {
         name = "spigotmc-repo"
         url = "https://hub.spigotmc.org/nexus/content/repositories/snapshots/"
@@ -15,10 +18,11 @@ repositories {
         name = "sonatype"
         url = "https://oss.sonatype.org/content/groups/public/"
     }
+
 }
 
 dependencies {
-    compileOnly("org.spigotmc:spigot-api:1.21.1-R0.1-SNAPSHOT")
+    compileOnly("io.papermc.paper:paper-api:1.21.1-R0.1-SNAPSHOT")
     implementation files("libs/PDK-1.4.0.jar")
 }