This can happen if the currency for the account doesn’t match the currency for the category.
For, example if I have categories in $, but an account in Euros, I cannot use the $ categories.
To fix this, go into Category Setup and check the currencies and in to Account Setup and make sure the currencies match. Or, you can create more categories for your alternate currency account.
We designed it this way because we can’t have a category being used in accounts that use both $ and Euro (for example) since the currency conversion is not 1:1.