That happens when your table prefix is long. It tries to create an index named table name + column name and unfortunately because that extension uses very long table and column names it's very close to the limit. I didn't hear anyone have that issue with that particular extension before though.
To work around this I either need to manually name the index and publish an extension update, you need to shorten your database table prefix or you need to manually create the tables and columns in the database then mark the migration as executed in the database 😅
The problem of manually naming the index in the extension itself is that either I hard-code it, but it can conflict with another Flarum install in the same database, or I just shorten it while preserving prefix but it might still cause errors for people with even longer prefixes...
My recommendation would be to simply install Flarum database without any prefix. There are practically no reasons to share a database between multiple apps nowadays.
