I have this pirate ship model, but I can’t upload it because its 700,000 triangles and the limit is 10,000 
However, I tried using the Decimate geometry feature to decrease the number of triangles, but it doesn’t work…
Could anyone help me, and if you could do it for me ill pay some robux (:
There is a limit to how much you can decimate, but in this case I believe you cant decimate because you imported the model, the lines are shining blue which means you got the model from somewhere else or you reimported it, for some reason blender doesn’t let you edit it sometimes.
I did get it from somewhere else, and even when I do ctrl A then do the decimate thing, it only make the triangles go down on some parts of the ship but some it stays the same.
I don’t think modifiers work on all of the selected parts at once. You would have to either join all of the parts of your object into one (which might cause some problems), or apply them one at a time for each part. You might also be able to link modifiers using ctrl + L, but I’m not completely sure. In addition, I would import the object as separate parts into studio as long as each part is under the 10k limit to have a bit more detail. If each part has over 10,000 tris, you probably have unneeded geometry and I recommend looking into the use of normal maps.
Did you try all decimate options? There’s 3 and they both decimate in different ways.
Just looking at the model I can tell that any modifier used arent going to work. As someone else mentioned, decimate or remesh only works to some extent. Essentially you only have two options, either you try to decimate as much as possible and get the tri count as low as possible without the model looking ‘funky’ then use the separate tool to separate the mesh into different sections making sure that the tri count isnt above 10k for each. You would then need to bulk import the model as fbx into studios. Make sure when you do this, you insert with location, this will make it so that every part mesh joinss back properly in studio. To summarize and give an example, say you manage to decimate the mesh in blender to apr 500k tris, you should split the mesh equally into 50 pieces which you’ll then bulk import into studio with location as fbx. It may sound time consuming but it really isnt since roblox allows bulk imports up to 200 meshs and since you’ll have 50 you should be able to do it in one go. The other option is simply to remodel the mesh with tracing to a low poly model. That option, however, will mean that you lose more detail on the original mesh.
it can only decimate so much, also make sure when you export the model you are applying the modifier otherwise nothing will change.
Decimate will only get you so far. I’m also not sure why you imported a 700 thousand tri model. If you insist on using this model, you will have to manually reduce triangles. I would split the model up into multiple pieces, and work on each piece individually.
Quickest solution, while not being the “preferred” way to do it, would be to separate the model into separate pieces and import each piece. With a mesh that has that many triangles you’re going to have way too many pieces though, so reducing triangles manually is your best bet.
If the model has multiple seperate parts than you can try joining some together with CTRL J while the parts you want to join are selected but this can cause problems if some parts have modifiers or are smooth , if so you’ll have to do each part manually.
However best way to delete extra verts, is by using the merge function, how you do it is by going into edit mode, click A then press M then “merge by distance”
