Unable to Assign Role with member.roles.add (id) - discord.js

Question

I'm trying to make an assignrole command which takes userID and roleID as arguments.
content = message.content.slice(message.content.indexOf(' ')+1)
args = content.split(', ');
const userID = args[0]
const roleID = args[1]
const desiredUser = message.guild.members.cache.get(userID)
const desiredRole = message.guild.roles.cache.get(roleID)
if (desiredUser != undefined){
if (desiredRole != undefined){
usern = desiredUser.user.username
desiredUser.roles.add(roleID).catch(console.error);
message.channel.send(usern + " has been assigned the "+ desiredRole.name +" role")
console.log("added role")
} else {
message.channel.send("role with ID " + roleID + " does not exist")
}
} else {
message.channel.send("user with ID " + userID + " does not exist")
}
The function succeeds at "getting" the correct user and role, and successfully sends the "USER has been assigned the ROLENAME role" message. However, the role itself isn't actually added to the user!
My console throws a DiscordAPIError: Missing Permissions error, but I don't know how to interpret it. The error does not quit the bot, however.
How do I add the role to the user?

Answer 1

It is likely that you are missing the MANAGE_ROLES permission as it is, like the name would suggest, required to manage rules.
https://discord.com/developers/docs/topics/permissions
Here are other possibilities that may be causing the error:
I found these here
It is trying to execute an action on a guild member with a role higher than or equal to your bots highest role.
It is trying to modify or assign a role that is higher than or equal to its highest role.
It is trying to execute a forbidden action on the server owner.