Purpur 1.21, other plugins are FAWE,Chunky,Coreprotect,Geyser&Floodgate, Map still loads, but does not seem to update correctly leaving big holes.

Error Message:
13:32:48 WARN]: [squaremap] Exception mapping chunk column starting at [-643, -768] in minecraft:the_end
java.util.concurrent.CompletionException: java.lang.NoClassDefFoundError: ca/spottedleaf/moonrise/common/util/ChunkSystem
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315) ~[?:?]
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320) ~[?:?]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1770) ~[?:?]
at net.minecraft.server.TickTask.run(TickTask.java:18) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:151) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:24) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1576) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:201) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:125) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1553) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1546) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:135) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.managedBlock(MinecraftServer.java:1505) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1627) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1337) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:332) ~[purpur-1.21.jar:1.21-2257-cb0f04b]
at java.base/java.lang.Thread.run(Thread.java:1583) ~[?:?]
Caused by: java.lang.NoClassDefFoundError: ca/spottedleaf/moonrise/common/util/ChunkSystem
at squaremap-paper-mc1.21-1.2.6 (1).jar/xyz.jpenilla.squaremap.paper.util.chunksnapshot.PaperChunkSnapshotProvider.lambda$asyncSnapshot$0(PaperChunkSnapshotProvider.java:37) ~[squaremap-paper-mc1.21-1.2.6 (1).jar:?]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768) ~[?:?]
... 14 more

Also it seems it is unable to generate the map based on world data, and ´can only update (badly and with holes) when a player loaded the chunks

squaremap 1.2.6 needs the latest Paper 1.21 build, 1.2.5 is needed for earlier builds but really you should just update your server. Purpur is not something squaremap explicitly supports but I would guess they have updated builds including the latest Paper changes as well.