Please check your server PHP version and update to version 7.4 if it's lower than 7.4. (You may check our server minimum requirements bottom of this article) This will solve the Avas Core plugin error.

Server Minimum Requirements

PHP version 7.4 or greater.
MySQL version 5.6 or greater OR MariaDB version 10.1 or greater.
WP Memory limit of 256 MB or greater
max_execution_time 360 (This needs to be increased if your server is slow and cannot import data.)
PHP Post Max Size: 256 MB or greater
Upload File Size: 256 MB
PHP Time Limit: 360
WordPress version 5.0 or greater.