forumPermisisons is a function defined in the functions_common.asp.
Make sure you have it. NOTE: it is misspelled! Leave it that way. If you fixed the misspelling on the function itself, you will get the error.
I checked the default install of v7.51 and it was misspelled there also.
The line you are getting the error on reads:
Call forumPermisisons(intForumID, intGroupID, intForumReadRights, intForumPostRights, intForumReplyRights, 0, 0, 0, 0, 0, 0, 0).
Just for fun, I changed the spelling on the call to forumPermissions and got this error:
Microsoft VBScript runtime (0x800A01F4)
Variable is undefined: 'forumPermissions'
/aspForum76/forum/default.asp, line 270
Not exactly the same as yours but similar.
So my 1st guess is you have a spelling problem.