Touching Cans on Step

Are robots allowed to touch the cans in the center of the field at the beginning of the match? like can I set a string attached to a hook inside the center cans before the match?

No.

Robots cannot start inside the landfill zone. So you couldn’t be touching the cans before the match.

G7-C

G7 When placed on the FIELD for a MATCH, each ROBOT must be:
A. in compliance with all ROBOT rules, i.e. it has passed Inspection. For exceptions regarding Practice MATCHES, see
Section 5.2 – Practice MATCHES.
B. fully supported by the floor, SCORING PLATFORM, and/or SCORING PLATFORM ramps on their ALLIANCE’S side of the
FIELD, and
C. completely outside of their AUTO ZONE and LANDFILL ZONE.
VIOLATION: If fix is a quick remedy, the MATCH won’t start until all requirements are met. If it is not a quick remedy the offending
ROBOT will be DISABLED and at the discretion of the Head REFEREE must be re-Inspected.