Is it okay to drink one beer and take a xanax (alprazolam)?

No: No. It is not a good idea. The combination of beer and a benzodiazepine or triazolobenzodiazepine is not a good one. There is compounded sedation between the two. You could inadvertently drink yourself under the table with just one beer. Then there are the risks of driving when intoxicated even if your blood alcohol level is low you can be arrested for driving under the intoxicating influence of two substances..