As it is stated above, it is not recommended to delete backup files manually, but let retention algorithm to handle that. However, if you need to free up some space to let backups complete successfully, you can remove older restore points manually, though they will still remain in the SQL configuration database as an available restore points (until flushed away by retention). If you are using reverse incremental backup mode, just delete the oldest .VRB files by their modified date. For forward incremental mode, you have to remove the oldest .VBK along with all the corresponding .VIB files.
To address space issues, it is recommended to configure built-in Veeam B&R space notifications or use one of the Veeam ONE predefined reports - Capacity Planning for backup repositories.