There are some problems that cause app to fail working. Not major issues. All those issues are fixable. Hope you find you solution here.
1. Check your internet connection
Check whether you have good internet connection and it is working. If you have install any internet data management tool please allow this to use internet or try by disabling that app.
2. Low Storage
If you are running out of storage space please backup some of your photos and other apps not used frequently and remove them from your phone and get some space. Then try using this app check whether it works smoothly as before.
3. Security permission issue.
If you have install any security or antivirus software then it may cause some problems. Go to that antivirus app and allow acmarket to use internet and storage permissions.
4. Check for latest version
Check you have latest version of this app has installed. Since you have to download this app by googling you may end up installing previous outdated app. Go download page to check whether you have latest version installed.
5. Reinstall app
Please uninstall and reinstall the app again. It may fix the problem. Always download latest version of this app store. If not try cleaning your device with Clean Master.