Check in your suppliers if you accidentally chose the inventory account instead of the expense account for the default expense account (supplier - options tab - expense account).  If you chose the inventory account by accident for your supplier, it won't let you use it in the inventory module.  For example, you may have chosen 1520 raw materials instead of 5020 expense raw material for one of your supplier's default expense account. Even though the message says linked account, this doesn't show up as a linked account.

Check in your suppliers if you accidentally chose the inventory account instead of the expense account for the default expense account (supplier - options tab - expense account).  If you chose the inventory account by accident for your supplier, it won't let you use it in the inventory module.  For example, you may have chosen 1520 raw materials instead of 5020 expense raw material for one of your supplier's default expense account. Even though the message says linked account, this doesn't show up as a linked account.