What is the request you are trying to make and how did you generate your access token? All the methods in the Catalog API are GET requests where the responses do not contain user data, so I believe you should be able to use app tokens for your Catalog API requests. Please follow the instructions here for generating your app token:
I too am seeing this issue. It only appears to work with the user tokens and not the application token. This documentation here seems directly implies that there are both application and user limits.
https://developer.ebay.com/api-docs/commerce/catalog/overview.html "**Rate limits** The Catalog API limits the number of calls that can be made per day on behalf of your application. For the current application rate limits, see the eBay API Call Limits page. The Catalog API also limits the number of calls that can be made by the authenticated user. Calls made on behalf of a given user are limited to 100,000 calls/day." But when making calls with the application token we get: "Insufficient permissions to fulfill the request"