I think this may be because the addon you tried to load/download maybe is unsupported by your version of Android. I presume that the specific addon is a .SOC file. If you play SRB2 Mobile online, I suggest you try to play on vanilla servers that have no addons, or just do not play multiplayer entirely and just stick to the main game. WADs and PK3s should work fine however, they did on my phone (Runs Android 10) but I don't know about .lua files. I think if you are going to play with addons, stray away from .lua and .SOC files. I hope this helped!
Note: No, i did not spell Lua wrong. It's Lua, not LUA. Unlike SOC files, Lua files are not an acronym. (SOC is short for Sonic Object Configuration) Just wanted to clear that up.