Acra Rail On This Bushmaster Help

That handguard isn't MLOK compatible. You can get a JP Enterprises ARCA rail that's rounded and then drill holes into the bottom of the tube to help mount it. You could also get a new free float rail that has the ARCA dovetails machined into it already or you could go with a FF MLOK rail and use a bolt on ARCA rail, for which there are many options.